Torsion resistant gap sub

ABSTRACT

A gap sub assembly for electromagnetic telemetry used in downhole drilling. The gap sub comprises a female part comprising a female mating section and a male part comprising a male mating section. The male mating section is matingly received within the female mating section and electrically isolated therefrom. One or more electrically insulating bodies secure the male part axially and torsionally relative to the female part. The electrically insulating bodies also electrically isolate the male part from the female part. The electrically insulating bodies can be installed through apertures in the female part or at least some of the electrically insulating bodies can be installed before the male mating section is inserted into the female mating section. The electrically insulating bodies can be held in place on the male mating section using a retention apparatus such as a ring, a scarf, pods or an adhesive.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. application Ser. No.15/752,737 now issued as U.S. patent Ser. No. 11/105,159, which is a 371of PCT application No. PCT/CA2016/050955 filed 12 Aug. 2016, whichclaims priority from U.S. application No. 62/205,549 filed 14 Aug. 2015,which are hereby incorporated herein by reference for all purposes. Forpurposes of the United States of America, this application claims thebenefit under 35 U.S.C. § 119 of U.S. application No. 62/205,549 filed14 Aug. 2015 and entitled TORSION RESISTANT GAP SUB.

TECHNICAL FIELD

This application relates to subsurface drilling, specifically to gap subassemblies suitable for use in measurement while drilling and methodsfor fabricating gap sub assemblies. Embodiments are applicable todrilling wells for recovering hydrocarbons.

BACKGROUND

Recovering hydrocarbons from subterranean zones typically involvesdrilling wellbores.

Wellbores are made using surface-located drilling equipment which drivesa drill string that eventually extends from the surface equipment to theformation or subterranean zone of interest. The drill string can extendthousands of feet or meters below the surface. The terminal end of thedrill string includes a drill bit for drilling (or extending) thewellbore. Drilling fluid, usually in the form of a drilling “mud”, istypically pumped through the drill string. The drilling fluid cools andlubricates the drill bit and also carries cuttings back to the surface.Drilling fluid may also be used to help control bottom hole pressure toinhibit hydrocarbon influx from the formation into the wellbore andpotential blow out at surface.

Bottom hole assembly (BHA) is the name given to the equipment at theterminal end of a drill string. In addition to a drill bit, a BHA maycomprise elements such as: apparatus for steering the direction of thedrilling (e.g. a steerable downhole mud motor or rotary steerablesystem); sensors for measuring properties of the surrounding geologicalformations (e.g. sensors for use in well logging); sensors for measuringdownhole conditions as drilling progresses; one or more systems fortelemetry of data to the surface; stabilizers; heavy weight drillcollars; pulsers; and the like. The BHA is typically advanced into thewellbore by a string of metallic tubulars (drill pipe).

Modern drilling systems may include any of a wide range ofmechanical/electronic systems in the BHA or at other downhole locations.Such electronic systems may be packaged as part of a downhole probe. Adownhole probe may comprise any active mechanical, electronic, and/orelectromechanical system that operates downhole. A probe may provide anyof a wide range of functions including, without limitation: dataacquisition; measuring properties of the surrounding geologicalformations (e.g. well logging); measuring downhole conditions asdrilling progresses; controlling downhole equipment; monitoring statusof downhole equipment; directional drilling applications; measuringwhile drilling (MWD) applications; logging while drilling (LWD)applications; measuring properties of downhole fluids; and the like.

A probe may comprise one or more systems for: telemetry of data to thesurface; collecting data by way of sensors (e.g. sensors for use in welllogging) that may include one or more of vibration sensors,magnetometers, inclinometers, accelerometers, nuclear particledetectors, electromagnetic detectors, acoustic detectors, and others;acquiring images; measuring fluid flow; determining directions; emittingsignals, particles or fields for detection by other devices; interfacingto other downhole equipment; sampling downhole fluids; etc.

A downhole probe may communicate a wide range of information to thesurface by telemetry. Telemetry information can be invaluable forefficient drilling operations. For example, telemetry information may beused by a drill rig crew to make decisions about controlling andsteering the drill bit to optimize the drilling speed and trajectorybased on numerous factors, including legal boundaries, locations ofexisting wells, formation properties, hydrocarbon size and location,etc. A crew may make intentional deviations from the planned path asnecessary based on information gathered from downhole sensors andtransmitted to the surface by telemetry during the drilling process. Theability to obtain and transmit reliable data from downhole locationsallows for relatively more economical and more efficient drillingoperations.

There are several known telemetry techniques. These include transmittinginformation by generating vibrations in fluid in the wellbore (e.g.acoustic telemetry or mud pulse (MP) telemetry) and transmittinginformation by way of electromagnetic signals that propagate at least inpart through the earth (EM telemetry). Other telemetry techniques usehardwired drill pipe, fibre optic cable, or drill collar acoustictelemetry to carry data to the surface.

Advantages of EM telemetry, relative to MP telemetry, include generallyfaster baud rates, increased reliability due to no moving downholeparts, high resistance to lost circulating material (LCM) use, andsuitability for air/underbalanced drilling. An EM system can transmitdata without a continuous fluid column; hence it is useful when there isno drilling fluid flowing. This is advantageous when a drill crew isadding a new section of drill pipe as the EM signal can transmitinformation (e.g. directional information) while the drill crew isadding the new pipe. Disadvantages of EM telemetry include lower depthcapability, incompatibility with some formations (for example, high saltformations and formations of high resistivity contrast), and some marketresistance due to acceptance of older established methods. Also, as theEM transmission is strongly attenuated over long distances through theearth formations, it requires a relatively large amount of power so thatthe signals are detected at surface. The electrical power available togenerate EM signals may be provided by batteries or another power sourcethat has limited capacity.

A typical arrangement for EM telemetry uses parts of the drill string asan antenna. The drill string may be divided into two conductive sectionsby including an insulating joint or connector (a “gap sub”) in the drillstring. The gap sub is typically placed at the top of a BHA such thatmetallic drill pipe in the drill string above the BHA serves as oneantenna element and metallic sections in the BHA serve as anotherantenna element. Gap subs may additionally or in the alternative beplaced at other locations along a drill string. EM telemetry signals canbe transmitted by applying electrical signals between the two antennaelements. The signals typically comprise very low frequency alternatingcurrent (AC) signals applied in a manner that codes information fortransmission to the surface or for transmission to another downholesystem. (Higher frequency signals attenuate faster than low frequencysignals.) The EM telemetry signals may be detected at the surface, forexample, by measuring electrical potential differences between the drillstring or a metal casing that extends into the ground and one or moreground rods.

Design of the gap sub is an important factor in an EM telemetry system.The gap sub must provide electrical isolation between two parts of thedrill string as well as withstand the extreme mechanical loading inducedduring drilling and the high differential pressures that occur betweenthe bore and exterior of the drill pipe. Drill string components aretypically made from high strength, ductile metal alloys in order tohandle the loading without failure. Many electrically-insulatingmaterials suitable for electrically isolating different parts of a gapsub (e.g. rubber, plastic, epoxy) are weaker than the metals generallyused to make downhole equipment. Other insulating materials (e.g.ceramics) are quite brittle. The mechanical properties of availableelectrically-insulating materials makes it difficult to design a gap subthat is both configured to provide efficient transmission of EMtelemetry signals and has the mechanical properties (e.g. the ability towithstand applied torques, axial forces, bending moments and shockloads) required of a link in the drill string.

In directional drilling, the trajectory of the wellbore may changerapidly, such as in building of a curve. In cases where the change indirection occurs more rapidly than planned or desired it can lead toharmful side effects within the section or “dogleg”. In such cases thecasing may not fit easily through the curved section. Repeated abrasionby the drill string in the particular location of the dogleg can resultin worn spots in which the BHA may become lodged. Excessive doglegs canalso increase the overall friction of the drill string, resulting inincreased potential for damage of the BHA. For components such as gapsubs which are generally weaker links in the drill string, the potentialfor damage and excessive wear of the electrically-insulating material isincreased by bends in the wellbore.

The reduced diameter of metal structural components passing throughexternal gaps in gap subs can cause the gap to act as a flex collarwhich can cause excessive stress in the external gap section whenundergoing bending. The dielectric material in this gap will usuallychip out, crack or buckle due to compressive loading, from wear in thewellbore, or from impact with the wellbore.

There remains a need for resilient, stiff, torsion resistant gap substhat are easy to install.

SUMMARY

The invention has a number of different aspects. These include, withoutlimitation:

-   -   methods for constructing gap sub assemblies;    -   gap sub assemblies in which electrically insulating bodies for        arresting relative torsional movement between a male part and a        female part can be installed before inserting a male part into a        female part;    -   gap sub components comprising features for facilitating        introduction of torque and/or tension resisting members;    -   apparatuses for securing spheres or other torsion-resisting        elements onto a male part before the male part is inserted into        the female part; and    -   intermediate fill plugs for preventing O-ring lubrication from        contaminating the interior of the gap sub assembly.

The various concepts described herein may be used in any combinations ormay be applied individually. These may be applied to make downholecomponents such as gap subs. For any of the described gap subconstructions which include a collar one can make similar devices thatlack a collar.

One example aspect provides a gap sub assembly comprising a male part, afemale part and an insulating collar. The male part may be secured tothe female part by providing electrically insulating bodies (e.g.spheres) that engage grooves or other indentations in the male part andfemale part. In particular, the grooves may include longitudinal groovesand helical grooves. The electrically insulating bodies may be insertedinto a gap through apertures in the male and/or female parts or may besecured to the male part before the male part is inserted into thefemale part. A high dielectric, nonconductive material may be injectedinto a radial gap between the external surface of the male part and theinternal surface of the female part.

Another example aspect provides securing apparatus for securingelectrically insulating bodies to a male part before the male part isinserted into a female part. In particular embodiments, the securingapparatus may comprise a ring having one or more channels for receivingand retaining the electrically insulating bodies. The ring may be slidonto the male part before or after the electrically insulating bodiesare received within the ring.

In other embodiments, the securing apparatus may comprise a flexiblestrip having one or more channels for receiving and retaining theelectrically insulating bodies. The flexible strip may be wrapped aroundthe male part before or after the electrically insulating bodies arereceived within the flexible strip.

In other embodiments, the securing apparatus may comprise one or morepods, wherein each pod has a single channel for receiving and retainingthe electrically insulating bodies. The pods may be installed into thegrooves of the male part before or after the electrically insulatingbodies are received within the pods.

In other embodiments, the securing apparatus may comprise an adhesivematerial for securing the electrically insulating bodies to the malepart. The adhesive may first be installed on the male part beforeinstalling the electrically insulating bodies or may be installed on theelectrically insulating bodies themselves before installing them on themale part.

Another exemplary aspect provides an intermediate plug for filling anaperture in a male or female part and preventing lubricant from a fillplug O-ring from penetrating into the gap sub. The aperture in the maleor female part may be used for inserting electrically insulating bodies.The intermediate fill plug may comprise a polymer plug having one ormore recesses for securing the intermediate plug. In some embodiments,the intermediate plug is integral to the fill plug.

The following are some non-limiting example enumerated embodiments whichillustrate various aspects of the invention.

-   1. A gap sub assembly comprising:

a female part having a female mating section and a male partelectrically isolated from the female part, the male part having a malemating section and a gap section, the male mating section being insertedinto a bore of the female mating section whereby the male and femalemating sections overlap in an axial direction and are spaced radiallyapart from one another to provide a radial gap between the male andfemale mating sections wherein the male mating section comprises a firstgenerally cylindrical male mating subsection having a first externaldiameter and one or more longitudinal grooves formed into an exteriorsurface thereof and a second generally cylindrical male matingsubsection having a second external diameter greater than the firstexternal diameter and one or more circumferentially-extending groovesformed into an exterior surface thereof with a step between the firstand second male mating subsections and the female mating sectioncomprises a first female mating subsection having a first internaldiameter greater than the first external diameter and one or morelongitudinal grooves formed into an interior surface thereof a secondfemale mating subsection having a second internal diameter greater thanthe second external diameter and the first internal diameter and one ormore circumferentially-extending grooves formed into an interior surfacethereof;

an electrically insulating collar positioned on the gap section;

a first plurality of electrically-insulating bodies located between theoverlapping male and female mating sections and spanning betweencorresponding ones of the longitudinal grooves in the first male andfemale mating subsections;

a second plurality of electrically-insulating bodies located between theoverlapping male and female mating sections and spanning betweencorresponding ones of the circumferentially-extending grooves in thesecond male and female mating subsections;

such that the male and female parts are mechanically coupled together.

-   2. A gap sub assembly according to enumerated example embodiment 1    (or any other example enumerated embodiment), wherein the one or    more longitudinal grooves of the male mating section are radially    aligned with corresponding ones of the one or more longitudinal    grooves of the female mating section.-   3. A gap sub assembly according to enumerated example embodiment 2    (or any other example enumerated embodiment), wherein the first    plurality of electrically-insulating bodies comprises a plurality of    first spheres.-   4. A gap sub assembly according to enumerated example embodiment 1    (or any other example enumerated embodiment) comprising a member    located in the radial gap that attaches together two or more of the    first plurality of electrically-insulating bodies.-   5. A gap sub assembly according to enumerated example embodiment 4    (or any other example enumerated embodiment), wherein the member    comprises a ring extending around the first male mating subsection    and the two or more of the first plurality of    electrically-insulating bodies are embedded in the ring.-   6. A gap sub assembly according to enumerated example embodiment 4    (or any other example enumerated embodiment), wherein the member    comprises a scarf comprising a sheet of flexible material wrapped    around the first male mating subsection and the two or more of the    first plurality of electrically-insulating bodies are attached to    the scarf.-   7. A gap sub assembly according to enumerated example embodiment 4    (or any other example enumerated embodiment), wherein the member    attaches together two or more of the electrically insulating bodies    that span between the same ones of the longitudinal grooves in the    first male and female mating subsections.-   8. A gap sub assembly according to any of enumerated example    embodiments 1 to 7 (or any other example enumerated embodiment),    wherein the one or more circumferentially-extending grooves of the    male mating section are radially aligned with corresponding ones of    the one or more circumferentially-extending grooves of the female    mating section.-   9. A gap sub assembly according to enumerated example embodiment 8    (or any other example enumerated embodiment) wherein the    circumferentially-extending grooves of the male and female mating    sections extend helically around surfaces of the male and female    mating sections respectively.-   10. A gap sub assembly according to any one of enumerated example    embodiments 1 to 9 (or any other example enumerated embodiment)    wherein the second plurality of electrically-insulating bodies    comprises a plurality of second spheres.-   11. A gap sub assembly according to any one of enumerated example    embodiments 1 to 10 (or any other example enumerated embodiment)    comprising a dielectric material filling the radial gap around the    first and second pluralities of electrically-insulating bodies.-   12. A gap sub assembly according to any of enumerated example    embodiments 1 to 11 (or any other example enumerated embodiment),    wherein the first plurality of electrically-insulating bodies    comprise ceramic bodies.-   13. A gap sub assembly according to any of enumerated example    embodiments 1 to 12 (or any other example enumerated embodiment),    wherein the second plurality of electrically-insulating bodies    comprise ceramic bodies.-   14. A gap sub assembly according to any of enumerated example    embodiments 1 to 13 (or any other example enumerated embodiment),    comprising first and second shoulders respectively on the male and    female parts at first and second ends of the gap section and wherein    the insulating collar is pre-loaded in compression to bear against    the first and second shoulders with a pre-load force.-   15. A gap sub assembly according to any of enumerated example    embodiments 1 to 13 (or any other example enumerated embodiment),    comprising a threaded ring mounted adjacent to the gap section, the    threaded ring operable to adjust a compression of the insulating    collar.-   16. A gap sub assembly according to any one of enumerated example    embodiments 1 to 15 (or any other example enumerated embodiment)    comprising one or more fill openings, each of the fill openings    extending from a surface of the gap sub to a corresponding one of    the circumferentially-extending grooves, the fill openings each    dimensioned to allow passage of a body of the second plurality of    bodies into the radial gap.-   17. A gap sub assembly according to enumerated example embodiment 16    (or any other example enumerated embodiment) wherein one or more of    the fill openings is closed by a fill plug comprising an elastomer    seal.-   18. A gap sub assembly according to enumerated example embodiment 17    (or any other example enumerated embodiment) wherein the elastomer    seal comprises an O-ring.-   19. A gap sub assembly according to enumerated example embodiment 17    or 18 (or any other example enumerated embodiment) comprising an    intermediate plug installed in the fill opening between the fill    plug and the corresponding one of the circumferentially-extending    grooves.-   20. A gap sub comprising:

a female part having a female mating section;

a male part electrically isolated from the female part, the male parthaving a male mating section and a gap section, the male mating sectionbeing inserted into a bore of the female mating section whereby the maleand female mating sections overlap in an axial direction and are spacedradially apart from one another to provide a radial gap between the maleand female mating sections wherein surfaces of the male mating sectionand female mating section facing one another across the radial gapcomprise corresponding grooves;

an electrically insulating collar positioned on the gap section;

a plurality of electrically-insulating bodies located between theoverlapping male and female mating sections and spanning betweencorresponding ones of the grooves in the surfaces of the male and femalemating sections such that the male and female parts are mechanicallycoupled together;

wherein at least one of the surfaces of the male mating section andfemale mating section facing one another across the radial gap is formedsuch that opposing edges of at least some of the grooves on the at leastone surface have different elevations.

-   21. A gap sub according to enumerated example embodiment 20 (or any    other example enumerated embodiment) wherein the surfaces of both    the male and female mating sections are formed such that opposing    edges of at least some of the grooves on each of the surfaces have    different elevations.-   22. A gap sub according to enumerated example embodiment 21 (or any    other example enumerated embodiment) wherein for at least some pairs    of the corresponding grooves a higher side of the groove on the male    mating section is radially outward relative to a higher side of the    groove on the female mating section.-   23. A gap sub according to enumerated example embodiment 22 (or any    other example enumerated embodiment) wherein a centroid of at least    one of the bodies that spans between the grooves of the pair of    corresponding grooves is radially between the higher side of the    groove on the male mating section and the higher side of the groove    on the female mating section.-   24. A gap sub according to any one of enumerated example embodiments    21 to 23 (or any other example enumerated embodiment) wherein for a    pair of corresponding grooves respectively on the male and female    mating sections higher and lower sides of the corresponding groove    on the male mating section are respectively radially aligned with    lower and higher sides of the corresponding groove on the female    mating section.-   25. A gap sub according to any one of enumerated example embodiments    20 to 24 (or any other example enumerated embodiment) wherein the    bodies comprise spherical balls.-   26. A gap sub according to any one of enumerated example embodiments    20 to 24 (or any other example enumerated embodiment) wherein the    bodies are cylindrical.-   27. A gap sub according to any one of enumerated example embodiments    20 to 24 (or any other example enumerated embodiment) wherein the    bodies comprise elongated rods.-   28. A gap sub according to enumerated example embodiment 27 (or any    other example enumerated embodiment) wherein the rods are segmented.-   29. A gap sub according to enumerated example embodiment 27 or 28    (or any other example enumerated embodiment) wherein the rods are    non-circular in cross-section.-   30. A gap sub according to enumerated example embodiment 29 (or any    other example enumerated embodiment) wherein the rods are polygonal    in cross section.-   31. A gap sub according to any one of enumerated example embodiments    20 to 30 (or any other example enumerated embodiment) wherein at    least some of the grooves extend substantially longitudinally along    the gap sub.-   32. A gap sub according to enumerated example embodiment 31 (or any    other example enumerated embodiment) wherein higher and lower sides    of the longitudinal grooves on the male mating section are arranged    such that for two adjacent ones of the grooves the higher sides of    the adjacent grooves are adjacent or the lower sides of the adjacent    grooves are adjacent.-   33. A gap sub according to enumerated example embodiment 31 (or any    other example enumerated embodiment) wherein higher and lower sides    of the longitudinal grooves on the male mating section are arranged    such that for two adjacent ones of the grooves the higher side one    of the adjacent grooves is adjacent to the lower sides of the other    adjacent groove.-   34. A gap sub according to any one of enumerated example embodiments    20 to 33 (or any other example enumerated embodiment) wherein at    least some of the grooves comprise pockets formed in one wall of the    groove, the pockets shaped to conform with surfaces of the bodies.-   35. A gap sub according to enumerated example embodiment 34 (or any    other example enumerated embodiment) wherein the pockets comprise    spherical cups spaced apart along the one wall of the groove.-   36. A gap sub according to any one of enumerated example embodiments    20 to 35 (or any other example enumerated embodiment) wherein bottom    surfaces of the grooves are arcuate.-   37. A gap sub according to any one of enumerated example embodiments    20 to 35 (or any other example enumerated embodiment) wherein the    grooves are V-shaped in cross section.-   38. A gap sub according to any one of enumerated example embodiments    20 to 37 wherein, for some or all of the plurality of bodies, the    body projects more deeply into one of the corresponding grooves than    into the other one of the corresponding grooves.-   39. A gap sub according to enumerated example embodiment 38 (or any    other example enumerated embodiment) wherein the body projects more    deeply into the corresponding groove on the male mating section than    into the corresponding groove on the female mating section.-   40. A gap sub according to any one of enumerated example embodiments    20 to 39 (or any other example enumerated embodiment) comprising a    longitudinal bore extending through the gap sub and threaded    couplings on opposing ends of the gap sub.-   41. A gap sub according to enumerated example embodiment 40 (or any    other example enumerated embodiment) wherein one of the threaded    couplings is a pin coupling comprising a male thread and one of the    threaded couplings is a box coupling comprising a female thread.-   42. A gap sub comprising:

a female part having a female mating section;

a male part electrically isolated from the female part, the male parthaving a male mating section and a gap section, the male mating sectionbeing inserted into a bore of the female mating section whereby the maleand female mating sections overlap in an axial direction and are spacedradially apart from one another to provide a radial gap between the maleand female mating sections wherein surfaces of the male mating sectionand female mating section facing one another across the radial gapcomprise corresponding grooves;

an electrically insulating collar positioned on the gap section;

a plurality of electrically-insulating bodies located between theoverlapping male and female mating sections and spanning betweencorresponding ones of the grooves in the surfaces of the male and femalemating sections such that the male and female parts are mechanicallycoupled together;

wherein for at least some pairs of the corresponding grooves on the maleand female mating sections the grooves are formed such that the bodyprojects more deeply into one of the corresponding grooves than theother one of the corresponding grooves.

-   43. A gap sub according to enumerated example embodiment 42 (or any    other example enumerated embodiment) wherein the body projects more    deeply into the corresponding groove on the male mating section than    into the corresponding groove on the female mating section.-   44. A gap sub according to any one of enumerated example embodiments    42 or 43 (or any other example enumerated embodiment) wherein the    bodies comprise spherical balls.-   45. A gap sub according to any one of enumerated example embodiments    42 or 43 (or any other example enumerated embodiment) wherein the    bodies are cylindrical.-   46. A gap sub according to any one of enumerated example embodiments    42 or 43 (or any other example enumerated embodiment) wherein the    bodies comprise elongated rods.-   47. A gap sub according to enumerated example embodiment 46 (or any    other example enumerated embodiment) wherein the rods are segmented.-   48. A gap sub according to enumerated example embodiment 46 or 47    (or any other example enumerated embodiment) wherein the rods are    non-circular in cross-section.-   49. A gap sub according to enumerated example embodiment 48 (or any    other example enumerated embodiment) wherein the rods are polygonal    in cross section.-   50. A gap sub according to any one of enumerated example embodiments    46 to 48 (or any other example enumerated embodiment) wherein a    portion of the rod projects into each of the corresponding grooves    and the corresponding grooves are shaped to conform with the shapes    of the surfaces of the respective portions of the rod.-   51. A gap sub according to any one of enumerated example embodiments    42 to 50 (or any other example enumerated embodiment) wherein the    bodies are non-circular in cross section and wherein the bodies are    oriented such that a major transverse axis of the bodies forms an    angle with a radius of the gap sub.-   52. A gap sub according to enumerated example embodiment 51 (or any    other example enumerated embodiment) wherein for a first group of    the bodies the major transverse axis of the bodies forms first angle    with a radius of the gap sub passing through a centroid of the body    and for a second group of the bodies the major transverse axis of    the bodies forms second angle with a radius of the gap sub passing    through a centroid of the body.-   53. A gap sub according to enumerated example embodiment 52 (or any    other example enumerated embodiment) wherein the first angle is    equal in magnitude and opposite in sign to the second angle.-   54. A gap sub according to any one of enumerated example embodiments    42 to 53 (or any other example enumerated embodiment) wherein at    least some of the grooves extend substantially longitudinally along    the gap sub.-   55. A gap sub according to any one of enumerated example embodiments    42 to 54 (or any other example enumerated embodiment) wherein at    least some of the grooves extend substantially circumferentially.-   56. A gap sub according to enumerated example embodiment 55 (or any    other example enumerated embodiment) wherein the grooves that extend    substantially circumferentially include helical portions.-   57. A gap sub according to any one of enumerated example embodiments    42 to 56 (or any other example enumerated embodiment) wherein at    least some of the grooves comprise pockets formed in one wall of the    groove, the pockets shaped to conform with surfaces of the bodies.-   58. A gap sub according to enumerated example embodiment 57 (or any    other example enumerated embodiment) wherein the pockets comprise    spherical cups spaced apart along the one wall of the groove.-   59. A gap sub according to enumerated example embodiment 56 or 57    (or any other example enumerated embodiment) wherein each of a    groove on the male mating section and a corresponding groove on the    female mating section is formed to include the pockets along one    wall thereof.-   60. A gap sub according to enumerated example embodiment 59 (or any    other example enumerated embodiment) wherein the pockets are formed    on opposite walls of the corresponding grooves on the male and    female mating sections.-   61. A gap sub according to enumerated example embodiment 59 or 60    (or any other example enumerated embodiment) wherein the    corresponding grooves on the male and female section that include    the pockets extend substantially longitudinally.-   62. A gap sub according to any one of enumerated example embodiments    57 to 60 (or any other example enumerated embodiment) wherein the    bodies in the corresponding grooves are engaged with the pockets    such that the bodies are prevented from moving along the    corresponding grooves.-   63. A gap sub according to any one of enumerated example embodiments    42 to 62 (or any other example enumerated embodiment) comprising a    longitudinal bore extending through the gap sub and threaded    couplings on opposing ends of the gap sub.-   64. A gap sub according to enumerated example embodiment 63 (or any    other example enumerated embodiment) wherein one of the threaded    couplings is a pin coupling comprising a male thread and one of the    threaded couplings is a box coupling comprising a female thread.-   65. A gap sub comprising:

a female part having a female mating section;

a male part electrically isolated from the female part, the male parthaving a male mating section and a gap section, the male mating sectionbeing inserted into a bore of the female mating section whereby the maleand female mating sections overlap in an axial direction and are spacedradially apart from one another to provide a radial gap between the maleand female mating sections wherein surfaces of the male mating sectionand female mating section facing one another across the radial gapcomprise corresponding grooves;

an electrically insulating collar positioned on the gap section;

a plurality of electrically-insulating bodies located between theoverlapping male and female mating sections and spanning betweencorresponding ones of the grooves in the surfaces of the male and femalemating sections such that the male and female parts are mechanicallycoupled together;

wherein at least some of the grooves comprise pockets formed in one wallof the groove, the pockets shaped to conform with surfaces of the bodiesreceived in the groove.

-   66. A gap sub according to enumerated example embodiment 65 (or any    other example enumerated embodiment) wherein the bodies are    spherical and the pockets comprise spherical cups spaced apart along    the one wall of the groove.-   67. A gap sub according to enumerated example embodiment 65 (or any    other example enumerated embodiment) wherein the bodies are    barrel-shaped and the pockets are shaped to conform with curved    surfaces of the bodies.-   68. A gap sub according to any one of enumerated example embodiments    65 to 67 (or any other example enumerated embodiment) wherein each    of a groove on the male mating section and a corresponding groove on    the female mating section is formed to include the pockets along one    wall thereof.-   69. A gap sub according to enumerated example embodiment 68 (or any    other example enumerated embodiment) wherein the pockets are formed    on opposite walls of the corresponding grooves on the male and    female mating sections.-   70. A gap sub according to enumerated example embodiment 67 or 68    (or any other example enumerated embodiment) wherein the    corresponding grooves on the male and female section that include    the pockets extend substantially longitudinally in the gap sub    passing through a centroid of the body.    71. A gap sub according to enumerated example embodiment 70 (or any    other example enumerated embodiment) wherein the bodies are    non-circular in cross section and wherein the bodies are oriented    such that a major transverse axis of the bodies forms an angle with    a radius of the gap sub.-   72. A gap sub according to enumerated example embodiment 71 (or any    other example enumerated embodiment) wherein for a first group of    the bodies the major transverse axis of the bodies forms first angle    with a radius of the gap sub passing through a centroid of the body    and for a second group of the bodies the major transverse axis of    the bodies forms second angle with a radius of the gap sub passing    through a centroid of the body.-   73. A gap sub according to enumerated example embodiment 72 (or any    other example enumerated embodiment) wherein the first angle is    equal in magnitude and opposite in sign to the second angle.-   74. A gap sub according to any one of enumerated example embodiments    65 to 73 (or any other example enumerated embodiment) wherein the    bodies in the corresponding grooves are engaged with the pockets    such that the bodies are prevented from moving along the    corresponding grooves.-   75. A gap sub according to any one of enumerated example embodiments    65 to 74 (or any other example enumerated embodiment) comprising a    longitudinal bore extending through the gap sub and threaded    couplings on opposing ends of the gap sub.-   76. A gap sub according to enumerated example embodiment 75 (or any    other example enumerated embodiment) wherein one of the threaded    couplings is a pin coupling comprising a male thread and one of the    threaded couplings is a box coupling comprising a female thread.-   77. A gap sub comprising:

a female part having a female mating section;

a male part electrically isolated from the female part, the male parthaving a male mating section and a gap section, the male mating sectionbeing inserted into a bore of the female mating section whereby the maleand female mating sections overlap in an axial direction and are spacedradially apart from one another to provide a radial gap between the maleand female mating sections wherein surfaces of the male mating sectionand female mating section facing one another across the radial gapcomprise corresponding grooves;

an electrically insulating collar positioned on the gap section;

a plurality of electrically-insulating bodies located between theoverlapping male and female mating sections and spanning betweencorresponding ones of the grooves in the surfaces of the male and femalemating sections such that the male and female parts are mechanicallycoupled together;

wherein at least some of the bodies are non-circular in cross sectionand the grooves in which the non-circular bodies are received are shapedto orient the bodies such that major diameters of the bodies have fixedorientations relative to the gap sub.

-   78. A gap sub according to enumerated example embodiment 77 (or any    other example enumerated embodiment) wherein the non-circular bodies    are received in grooves that extend generally longitudinally    relative to the gap sub.-   79. A gap sub according to enumerated example embodiment 78 (or any    other example enumerated embodiment) wherein the bodies are oriented    such that a major transverse axis of the bodies forms an angle with    a radius of the gap sub that passes through the centroid of the    body.-   80. A gap sub according to enumerated example embodiment 79 (or any    other example enumerated embodiment) wherein for a first group of    the bodies the major transverse axis of the bodies forms a first    angle with a radius of the gap sub passing through a centroid of the    body and for a second group of the bodies the major transverse axis    of the bodies forms a second angle with a radius of the gap sub    passing through a centroid of the body.-   81. A gap sub according to enumerated example embodiment 80 (or any    other example enumerated embodiment) wherein the first angle is    equal in magnitude and opposite in sign to the second angle.-   82. A gap sub according to enumerated example embodiment 80 or 81    (or any other example enumerated embodiment) wherein the bodies of    the first group of bodies are arranged in rows extending    longitudinally along a first plurality of the generally longitudinal    grooves and the bodies of the second group of bodies are arranged in    rows extending longitudinally along a second plurality of the    generally longitudinal grooves.-   83. A gap sub according to enumerated example embodiment 82 (or any    other example enumerated embodiment) wherein the grooves containing    the bodies of the first group of bodies alternate in a    circumferential direction with the grooves containing the bodies of    the second group of bodies.-   84. A gap sub according to any one of enumerated example embodiments    79 to 83 (or any other example enumerated embodiment) wherein the    angle is in the range of 10 to 70 degrees.-   85. A gap sub according to any one of enumerated example embodiments    77 to 84 (or any other example enumerated embodiment) wherein the    bodies comprise elongated rods.-   86. A gap sub according to enumerated example embodiment 85 (or any    other example enumerated embodiment) wherein the rods are segmented.-   87. A gap sub according to enumerated example embodiment 85 or 86    (or any other example enumerated embodiment) wherein cross sections    of the rods have flat facets.-   88. A gap sub according to any one of enumerated example embodiments    85 to 87 (or any other example enumerated embodiment) wherein a    portion of the rod projects into each of the corresponding grooves    and the corresponding grooves are shaped to conform with the shapes    of the surfaces of the respective portions of the rod.-   89. A gap sub comprising:

a female part having a female mating section;

a male part electrically isolated from the female part, the male parthaving a male mating section and a gap section, the male mating sectionbeing inserted into a bore of the female mating section whereby the maleand female mating sections overlap in an axial direction and are spacedradially apart from one another to provide a radial gap between the maleand female mating sections wherein surfaces of the male mating sectionand female mating section facing one another across the radial gapcomprise corresponding grooves;

an electrically insulating collar positioned on the gap section;

a plurality of electrically-insulating bodies located between theoverlapping male and female mating sections and spanning betweencorresponding ones of the grooves in the surfaces of the male and femalemating sections such that the male and female parts are mechanicallycoupled together;

wherein at least some of the bodies comprise cylindrical bodies alignedwith the corresponding grooves.

-   90. A gap sub according to enumerated example embodiment 89 (or any    other example enumerated embodiment) wherein the cylindrical bodies    are circular in cross section.-   91. A gap sub according to enumerated example embodiment 89 (or any    other example enumerated embodiment) wherein the cylindrical bodies    are rectangular in cross section.-   92. A gap sub according to enumerated example embodiment 91 (or any    other example enumerated embodiment) wherein the cylindrical bodies    are square in cross section.-   93. A gap sub according to enumerated example embodiment 89 (or any    other example enumerated embodiment) wherein the cylindrical bodies    are elliptical in cross section.-   94. A gap sub according to enumerated example embodiment 89 (or any    other example enumerated embodiment) wherein the cylindrical bodies    have a bow-tie shape in cross section.-   95. A gap sub according to enumerated example embodiment 94 (or any    other example enumerated embodiment) wherein the grooves are    dovetail grooves.-   96. A gap sub according to any one of enumerated example embodiments    89 to 95 (or any other example enumerated embodiment) wherein the    cylindrical bodies are curved to match a curvature of the    corresponding grooves.-   97. A gap sub according to any one of enumerated example embodiments    89 to 95 (or any other example enumerated embodiment) wherein the    cylindrical bodies comprise elongated rods.-   98. A gap sub according to enumerated example embodiment 97 (or any    other example enumerated embodiment) wherein the rods comprise    weakened sections spaced longitudinally apart along the rods.-   99. A gap sub according to any one of enumerated example embodiments    89 to 98 (or any other example enumerated embodiment) wherein the    bodies comprise a ceramic material.-   100. A gap sub assembly comprising:

a female part having a female mating section, the female mating sectionhaving one or more longitudinal grooves formed into the interiorthereof;

a male part having a male mating section and a gap section, the malemating section having one or more longitudinal grooves formed into theexterior thereof and the male mating section being inserted into thefemale mating section whereby the male and female mating sectionsoverlap;

an insulating collar positioned on the gap section;

one or more longitudinal gaps defined by the longitudinal grooves formedinto the male and female sections;

one or more electrically isolating spheres located within one or more ofthe one or more longitudinal gaps such that the male and female partsare mechanically coupled together but electrically isolated from eachother at their mating sections, the spheres held in place at least inpart by a torsional ball channel ring; wherein the torsional ballchannel ring comprises a tubular sleeve having one or more sleevesformed therein, the one or more sleeves each configured to receive oneor more electrically insulating bodies.

-   101. A gap sub assembly comprising:

a female part having a female mating section, the female mating sectionhaving one or more longitudinal grooves formed into the interiorthereof;

a male part having a male mating section and a gap section, the malemating section having one or more longitudinal grooves formed into theexterior thereof and the male mating section being inserted into thefemale mating section whereby the male and female mating sectionsoverlap;

an insulating collar positioned on the gap section thereby electricallyisolating the male part from the female part;

one or more longitudinal gaps defined by the longitudinal grooves formedinto the male and female sections;

one or more electrically isolating bodies located within one or more ofthe one or more longitudinal gaps such that the male and female partsare mechanically coupled together but electrically isolated from eachother at their mating sections, the electrically insulating bodies heldin place at least in part by a torsional ball channel scarf, thetorsional ball channel scarf wrapped around at least a portion of themale mating section;

wherein the torsional ball channel scarf comprises a flexible generallyrectangular body having one or more sleeves formed therein, the one ormore sleeves each configured to receive one or more electricallyinsulating bodies.

-   102. A gap sub assembly comprising:

a female part having a female mating section, the female mating sectionhaving one or more longitudinal grooves formed into the interiorthereof;

a male part having a male mating section and a gap section, the malemating section having one or more longitudinal grooves formed into theexterior thereof and the male mating section being inserted into thefemale mating section whereby the male and female mating sectionsoverlap;

an insulating collar positioned on the gap section thereby electricallyisolating the male part from the female part;

one or more longitudinal gaps defined by the longitudinal grooves formedinto the male and female sections;

one or more electrically isolating bodies located within one or more ofthe one or more longitudinal gaps such that the male and female partsare mechanically coupled together but electrically isolated from eachother at their mating sections, the electrically insulating bodies heldin place at least in part by one or more torsional ball channel pods;

wherein each torsional ball channel pod comprises a sleeve configured toreceive one or more electrically insulating bodies.

-   103. A method for making a gap sub, the method comprising:

placing an insulating collar around a gap section of a male part;

installing one or more first electrically insulating bodies into each ofone or more longitudinal channels formed into an exterior of a portionof the male part;

inserting the male part into a female part such that a protrudingportion of each of the one more first electrically insulating bodiesengages one or more longitudinal channels formed into an interior of thefemale part; and

securing the male part to the female part so as to prevent relativeaxial movement of the male part with respect to the female part.

-   104. A method for making a gap sub according to enumerated example    embodiment 103, wherein securing the male part to the female part    comprises installing one or more second electrically insulating    bodies into a helical channel formed between a helical channel on    the exterior of the male part and a helical channel on the interior    of the female part.-   105. A method for making a gap sub according to any of enumerated    example embodiments 103 and 104, comprising compressing the    insulating collar.-   106. A method for making a gap sub according to enumerated example    embodiment 105 wherein compressing the insulating collar comprises    threading a threaded ring, installed between a shoulder of the male    part and the insulating collar, toward the insulating collar.-   107. A method for making a gap sub according to enumerated example    embodiment 105 wherein compressing the insulating collar comprises    threading a threaded ring, installed between a shoulder of the    female part and the insulating collar, toward the insulating collar.-   108. A method for making a gap sub according to any of enumerated    example embodiments 103 to 105, wherein installing one or more first    electrically insulating bodies into each of the one or more    longitudinal channels formed into an exterior of the male part    comprises inserting the one or more first electrically insulating    bodies into one or more channels of a torsional ball channel    apparatus.-   109. A method for making a gap sub according to enumerated example    embodiment 108, wherein the torsional ball channel apparatus    comprises an adhesive material.-   110. A method for making a gap sub according to enumerated example    embodiment 105, wherein the torsional ball channel apparatus    comprises a torsional ball channel ring, wherein the torsional ball    channel ring comprises a tubular sleeve having one or more channels    formed therein and the one or more channels are each configured to    receive one or more first electrically insulating bodies.-   111. A method for making a gap sub according to enumerated example    embodiment 105, wherein the torsional ball channel apparatus    comprises a torsional ball channel scarf, wherein the torsional ball    channel scarf comprises a flexible generally rectangular body having    one or more channels formed therein and the one or more channels are    each configured to receive one or more first electrically insulating    bodies.-   112. A method for making a gap sub according to enumerated example    embodiment 105, wherein the torsional ball channel apparatus    comprises one or more torsional ball channel pods, wherein each    torsional ball channel pod comprises a channel configured to receive    one or more first electrically insulating bodies.-   113. A method for making a gap sub according to any of enumerated    example embodiments 105 to 112, wherein the torsional ball channel    apparatus is installed on the male mating section before the one or    more first electrically insulating bodies are installed in the one    or more channels of the torsional ball channel apparatus.-   114. A method for making a gap sub according to any of enumerated    example embodiments 105 to 113, wherein the torsional ball channel    apparatus is installed on the male mating section after the one or    more electrically insulating bodies are installed in the one or more    channels of the torsional ball channel apparatus.-   115. A method for making a gap sub according to any of enumerated    example embodiments 105 to 114, wherein the male mating section and    the female mating section are dimensioned such that there is a    radial gap between the male mating section and the female mating    section when the male mating section is inserted into the female    mating section.-   116. A method for making a gap sub according to enumerated example    embodiment 115, comprising inserting a dielectric material into the    radial gap.-   117. A method for making a gap sub according to enumerated example    embodiment 115, comprising inserting a fill plug into the opening in    the gap sub.-   118. A method for making a gap sub according to enumerated example    embodiment 117, wherein the fill plug comprises a lubricated o-ring.-   119. A method for making a gap sub according to enumerated example    embodiment 118, comprising inserting an intermediate plug into the    opening in the gap sub before inserting the fill plug, wherein the    intermediate plug forms a barrier to prevent lubricant from passing.-   120. A method for making a gap sub according to any of enumerated    example embodiments 118 and 119, wherein one or more surfaces of the    intermediate plug define one or more recesses.-   121. A method for making a gap sub according to any of enumerated    example embodiments 103 to 120, wherein the one or more first    electrically insulating bodies comprise ceramic electrically    insulating spheres.-   122. A method for making a gap sub according to any of enumerated    example embodiments 104 to 121, wherein the one or more second    electrically insulating bodies comprise ceramic electrically    insulating spheres.

Further aspects of the invention and features of example embodiments areillustrated in the accompanying drawings and/or described in thefollowing description.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate non-limiting example embodiments ofthe invention.

FIG. 1 is a schematic view of a drilling operation.

FIG. 2 is a partially exploded cross-sectional side view of an exemplarygap sub assembly.

FIG. 3 is a cross-sectional side view of an exemplary gap sub assembly.

FIG. 4 is a partially exploded side view of an exemplary gap subassembly.

FIG. 5 is a cross-section of a portion of an exemplary gap sub assembly.

FIG. 6 is a perspective view of an exemplary gap sub assembly.

FIG. 7 is a detail view of a portion of an exemplary gap sub assembly,wherein the female part is depicted as translucent.

FIG. 8 is a cross-section of an exemplary gap sub assembly, wherein thefemale part is depicted as translucent.

FIG. 9 is a perspective view of an exemplary torsional ball channel ringbeing installed on a portion of an exemplary gap sub assembly.

FIG. 10 is a perspective view of an exemplary torsional ball channelring installed on a portion of an exemplary gap sub assembly.

FIG. 11 is a perspective view of an exemplary torsional ball channelscarf.

FIG. 12 is a perspective view of an exemplary torsional ball channelscarf installed on a portion of an exemplary gap sub assembly.

FIG. 13 is a perspective view of exemplary torsional ball channel podsbeing installed on a portion of an exemplary gap sub assembly.

FIG. 14 is a perspective view of exemplary torsional ball channel podsinstalled on a portion of an exemplary gap sub assembly.

FIG. 15 is a side view of a portion of an exemplary gap sub assembly.

FIG. 16 is a perspective view of a portion of an exemplary gap subassembly.

FIG. 17 is a cross-section of a portion of an exemplary gap subassembly.

FIG. 18 is a detail view of a portion of an exemplary gap sub assembly.

FIG. 19A is a perspective view of an exemplary fill plug.

FIG. 19B is a cross sectional view of an exemplary fill plug andintermediate plug installed in an opening of an exemplary gap sub.

FIGS. 19C and 19D are schematic diagrams of exemplary intermediateplugs.

FIG. 20A illustrates a portion of an exemplary male part.

FIG. 20B illustrates a portion of an exemplary female part.

FIG. 20C is a cross-section of a portion of an exemplary gap subassembly.

FIG. 20D is a cross-section of a portion of an exemplary gap subassembly.

FIG. 21A is a perspective view of exemplary bodies having a squarecross-section installed on a portion of an exemplary male part.

FIG. 21B is a perspective view of exemplary bodies having a roundcross-section installed on a portion of an exemplary male part.

FIGS. 22A through 22F are schematic cross-sectional views throughrod-like bodies of various cross-sectional shapes engaged to span a gapbetween channels having various cross-sectional configurations.

FIG. 23 is a cross-section of a portion of an exemplary gap subassembly.

FIG. 24 is an expanded cross-section of a portion of an exemplary gapsub assembly.

FIG. 25 is a cross-section of an exemplary gap sub assembly groovepattern.

FIG. 26 is a cross-section of a portion of an exemplary gab subassembly.

DESCRIPTION

Throughout the following description specific details are set forth inorder to provide a more thorough understanding to persons skilled in theart. However, well known elements may not have been shown or describedin detail to avoid unnecessarily obscuring the disclosure. The followingdescription of examples of the technology is not intended to beexhaustive or to limit the system to the precise forms of any exampleembodiment. Accordingly, the description and drawings are to be regardedin an illustrative, rather than a restrictive, sense.

FIG. 1 shows schematically an example drilling operation. A drill rig 10drives a drill string 12 which includes sections of drill pipe thatextend to a drill bit 14. The illustrated drill rig 10 includes aderrick 10A, a rig floor 10B and draw works 10C for supporting the drillstring. Drill bit 14 is larger in diameter than the drill string abovethe drill bit. An annular region 15 surrounding the drill string istypically filled with drilling fluid. The drilling fluid is pumpedthrough a bore in the drill string to the drill bit and returns to thesurface through annular region 15 carrying cuttings from the drillingoperation. As the well is drilled, a casing 16 may be made in the wellbore. A blow out preventer 17 is supported at a top end of the casing.The drill rig illustrated in FIG. 1 is an example only. The methods andapparatus described herein are not specific to any particular type ofdrill rig.

One or more gap subs 20 may be positioned at desired locations alongdrill string 12, for example, at the top of BHA 2. Gap sub 20 provideselectrical isolation between two electrically-conductive parts of thedrill string respectively located above and below the gap sub. The twoparts form a dipole antenna structure. For example, one part of thedipole may be made of BHA 2 up to the electrically insulating gap suband the other part of the dipole may be made up of the part of the drillstring extending from the gap sub toward the surface.

A very low frequency alternating current (AC) electrical signal 19A isgenerated by an EM telemetry signal generator 18 and applied across gapsub 20. The low frequency AC signal energizes the earth and creates anelectrical field 19B which results in a measurable voltage differentialat a telemetry receiver. The voltage differential may, for example, bebetween the top of drill string 12 and one or more grounded electrodes13B (such as ground rods or ground plates). Electrical signal 19A isvaried in a way which encodes information for transmission by telemetry.

Some aspects disclosed herein provide improved gap sub assemblies. Somegap sub assemblies disclosed herein provide improved characteristics forresisting torques, axial forces and/or bending moments. Some gap subassemblies disclosed herein provide reduced potential for leaking (e.g.have fewer openings for receiving electrically-insulating bodies) andare easier to assemble. Other aspects disclosed herein provide methodsfor manufacturing gap sub assemblies. Some methods for manufacturing gapsub assemblies disclosed herein provide systems for more efficientinstallation of electrically-insulating bodies. These aspects may beapplied individually or in combination.

The gap sub assembly embodiments described herein generally relate togap sub assemblies that may be used for EM telemetry in downholedrilling. In some embodiments the gap sub assembly comprises a femalepart comprising a female mating section and a male part comprising amale mating section and a gap section. The male mating section ismatingly received within the female mating section and electricallyisolated therefrom. The gap section is electrically insulating overall.

In some embodiments, a collar is positioned on the gap section (see e.g.gap section 171 in FIG. 2 ) and supported between two parts of the gapsub assembly. Any suitable type of collar may be employed in conjunctionwith the gap sub described herein. For example, in some embodiments, thecollar may comprise one or more members that extend circumferentiallyaround the gap sub and are supported by a plurality of discrete bodies.Various examples of such collars are described in WO 2014/075190. Thecircumferential members may comprise rings. In a non-limiting exampleembodiment, the rings are metal rings and the discrete bodies compriseceramic spheres. The rings and discrete bodies may be embedded in anelectrically-insulating material. The rings may be shaped to providerecesses to receive the discrete bodies.

The collar therefore electrically isolates the male part from the femalepart. The male part, female part and insulating collar function as the“gap sub” for EM telemetry. The male part and female part may eachcomprise a suitable coupling (e.g. an API standard threaded coupling)for coupling the gap sub to uphole and downhole parts of the drillstring.

It is desirable to provide gap subs that are designed in a way thatfacilitates manufacture and also provides good electrical insulationbetween uphole and downhole ends of the gap sub. In some embodiments agap sub is made by placing a collar between male and female parts andthen mating the male and female parts. Once the collar is positioned onthe gap section, the female part can be mated with the male part to formthe gap sub assembly. Where the collar will be compressively pre-loadedthen, depending on the mechanism for applying the pre-loading, thepre-loading may be performed before, after or as part of the mating ofthe male section to the female section. A suitable dielectric materialmay then be applied to fill the spaces around the collar.

Providing a collar that is compressed can increase resistance of the gapsection to bending. Essentially, the collar may carry forces betweenshoulders of the male and female parts thereby resisting bending of thegap sub. The collar functions in place of solid material that would bepresent in a section of drill string lacking a gap section. A gapsection which includes a collar may approximate the resistance tobending of an equivalent section of drill string. In some embodiments,the section of drill string having a collar has a Young's modulus whichcan be greater than and, in different example embodiments, is at least150%, 120%, 110%, 100%, 99%, 95%, 90%, 80%, 70%, or 50% of the Young'smodulus of an equivalent section of drill string that does not have agap section. An equivalent section of drill string may comprise asection of drill string with the same material, outer diameter and borediameter as the gap sub assembly but made of solid metal.

A female part of a gap sub may be mated to the male part of a gap sub invarious ways. For example, the male part may be held to the female partby providing electrically-insulating bodies (e.g. spheres) that engagegrooves or other indentations in the male part and female part. Theelectrically-insulating bodies may be inserted into a gap throughapertures in the male or female part. Example embodiments having thisconstruction are discussed below and illustrated in FIGS. 2-18 .

One aspect of the invention provides a torsion-resistant gap sub 120.Gap sub 120 comprises a male part 130 and a female part 140. In someembodiments, a collar 170 is supported between male part 130 and femalepart 140. Male part 130 comprises two sets of grooves 134, 136 in thesurface of mating part 138. Female part 140 comprises two correspondingsets of grooves 144, 146 in the surface of mating part 148.

Grooves 134, 144, may interact with electrically-insulating bodies tosecure male part 130 longitudinally (axially) in the mated relationshiprelative to female part 140 while grooves 136, 146 may interact withother electrically-insulating bodies to secure male part 130 torsionallyin the mated relationship relative to female part 140. It should beunderstood that a gap sub may be made with grooves like grooves 134, 144and some torque-resisting arrangement not involving grooves 136, 146 andthat a gap sub may be made with grooves 136, 146 and some arrangementother than grooves 134, 144 may be provided for securing male part 130axially relative to female part 140.

Grooves 134, 144 may be helical or circumferential and are configured toreceive one or more electrically-insulating bodies. In the illustratedembodiment grooves 134, 144 are helical. This is advantageous as thehelical grooves provide one way to pre-load a collar (as described inmore detail elsewhere herein).

In the illustrated embodiment the electrically-insulating bodiescomprise spheres 150. For example, spheres 150 may be fed through anopening 154 into a space 152 which spans between grooves 134, 144.Opening 154 may be located at a first end of helical grooves 134, 144.As spheres 150 are introduced into opening 154, spheres located in space152 are pushed toward a second end of helical grooves 134, 144. Opening154 may be capped or closed after spheres 150 have been inserted.Opening 154 may comprise a fill port, as will be described in moredetail below. As depicted in FIG. 8 , opening 154 may be located infemale part 140. Alternatively, an opening or aperture could be locatedin male part 130, thus requiring spheres 150 to be inserted from withinthe bore of gap sub 120.

Grooves 134 may wind around the exterior circumference of mating part138 multiple times and grooves 144 may wind around the interiorcircumference of mating part 148 multiple times. Increasing the numberof windings of grooves 134, 144 around mating parts 138, 148 providesadditional rigidity and strength in the mating connection between malepart 130 and female part 140. In some embodiments, grooves 134, 144 windaround mating parts 138, 148 between one and three times, in otherembodiments, grooves 134, 144 wind around mating parts 138, 148 morethan three times. The number of windings may be based at least in parton the required strength of the coupling joint and/or the strength ofthe insulating bodies. Two or more sets of grooves 134, 144 may beprovided in some embodiments.

As illustrated in FIG. 2 , female part 140 comprises shoulder 142 andmale part 130 comprises shoulder 132. A gap between shoulders 132 and142 may be filled with a material or structure that is electricallyinsulating. In the current example embodiment the gap is filled with acollar 170 that provides a framework to support anelectrically-insulating material that fills gaps in the collar.

It can be appreciated that with spheres 150 in place, as described,twisting female part 140 with respect to male part 130 will result inshoulder 132 moving axially relative to shoulder 142. Thus collar 170may be axially compressed between shoulders 132 and 142 by suchrotation. The amount of compression can be adjusted by rotating malepart 130 more or less in relation to female part 140. Axiallycompressing collar 170 may be advantageous, as described above.

In some embodiments, axial compression of collar 170 is provided by athreaded ring 137 that can be turned to adjust its own position relativeto the position of shoulder 132 on male part 130 and relative to theposition of shoulder 142 on female part 140. The threaded ring mayengage threads on male part 130 or female part 140. In some embodimentsthe threads comprise acme threads.

FIGS. 2 and 3 depict example embodiments which include a threaded ring137 threadedly engaged with male part 130. By adjusting the position ofthreaded ring 137 relative to shoulder 132 and shoulder 142 (i.e.rotating threaded ring 137 relative to male part 130), compression ofcollar 170 may be increased or decreased. For example, if threaded ring137 is located on male part 130, adjacent shoulder 132 and opposed fromshoulder 142 and is adjusted toward shoulder 142, axial compression ofcollar 170 is increased. Conversely, if threaded ring 137 is located onfemale part 140, adjacent shoulder 142 and opposed from shoulder 132 andis adjusted toward shoulder 132, axial compression of collar 170 isincreased.

Adjacent turns of grooves 134, 144 can be spaced apart by variousdistances. As the spacing between adjacent turns of grooves 134, 144decreases, a greater number of spheres 150 can be inserted in a fixedlength of gap sub. Increasing the number of spheres 150 decreases thestress on each individual sphere 150 but may weaken section 138A due tothe reduction of material in section 138A. The spacing between turns maybe determined at least in part by cost effectiveness, strength ofspheres 150, a need to prevent shearing of grooves 134, 144 and a desireto allow material injected into the gap to fill all spaces within gap.In some embodiments, the spacing distance is greater or equal to thewidth of grooves 134, 144.

Grooves 134, 144 can have various cross-sectional shapes. Grooves 134,144 may have any of at least the following cross-sections: rounded,circle segment, angular, rectangular, square, trapezoidal andtriangular.

Spheres 150 can have any suitable diameter. Typically, all spheres 150are of the same size, although this is not mandatory. Spheres 150 may besized and shaped to correspond with the cross-sectional shapes ofgrooves 134, 144. The size of spheres 150 may be based at least in parton one or more of the cost of spheres 150, the shear strength of spheres150, the size of the gap sub and the potential presence of defects inspheres 150.

Spheres 150 (or other bodies for insertion into grooves 134, 1441 may bemade of an electrical insulator material, for example, but not limitedto, ceramic, plastic, plastic coated metals, composite or carbides.Exemplary ceramics include, but are not limited to, zirconium dioxide,yttria tetragonal zirconia polycrystal (YTZP), silicon carbide, orcomposites. In other embodiments, spheres 150 are made of a metal ormetal alloy, for example, but not limited to, copper, copper alloys,aluminium or stainless steel. In embodiments where spheres 150 are notthemselves electrically-insulating one or more electrical insulators areprovided such that overall the gap provides electrical isolation of malepart 130 from female part 140. For example, if spheres 150 are made of ametal or metal alloy, an additional coating, such as a ceramic coating,on spheres 150 may be provided to aid in electrically isolating malepart 130 from female part 140.

Longitudinal grooves 136, 146 may be located so that a groove 136 isaxially aligned with a corresponding groove 146 when collar 170 has beenpre-loaded in compression to a desired amount. With grooves 136 and 146so aligned, electrically-insulating bodies may be introduced into aspace 162 that spans between a groove 136 and the corresponding groove146. In the currently illustrated embodiment theseelectrically-insulating bodies comprise spheres 160. The spheres 160 maybe introduced, for example, by way of openings that may be capped orclosed after the spheres 160 are in place. The openings may be locatedin female part 140. Alternatively, one or more openings could be locatedin male part 130 in which case spheres 160 may be inserted from withingap sub 120. In other embodiments, the spheres or otherelectrically-insulating bodies may be loaded into grooves 136 or 146before male part 130 is inserted into female part 140. In such otherembodiments, openings 154 are optional.

By inserting spheres 160 or other bodies into space 162 spanningindentations (e.g. grooves 136, 146) in the male and female parts 130,140, male part 130 is secured torsionally in relation to female part140. Securing male part 130 and female part 140 torsionally ensures thatthe desired amount of pre-loading is maintained, that male part 130 isnot able to unthread from female part 140 and that the torsionalrigidity of drill string 12 is not unduly compromised by gap sub 120.Spheres 160 also provide additional electrical insulation between malepart 130 and female part 140.

Longitudinal grooves 136, 146 may be of any suitable length. In someembodiments, all grooves 136, 146 are the same length. In otherembodiments, grooves 136, 146 may be of various lengths. Increasing thelength of grooves 136, 146 allows for additional spheres 160 to beinserted in grooves 136, 146 thereby reducing the stress on eachindividual sphere 160. Decreasing the length of grooves 136, 146 reducesmanufacturing costs and time.

Adjacent grooves 136, 146 can be spaced apart by various distances. Asthe circumferential (angular) spacing between adjacent grooves 136, 146decreases, a greater number of spheres 160 can be inserted in a fixedlength of gap sub. Increasing the number of spheres 160 decreases thestress on each individual sphere 160 but may weaken section 138B due tothe reduction of material in section 138B. The circumferential spacingbetween adjacent grooves 136, 146 may be determined at least in part bycost effectiveness, strength of spheres 160, a need to prevent shearingof grooves 136, 146 and a desire to allow material injected into the gapto fill the gap. In some embodiments, the spacing distance is greater orequal to the width of grooves 136, 146.

Grooves 134, 144 and grooves 136, 146 can have various cross-sectionalshapes. In some embodiments, grooves 134, 144 and grooves 136, 146 havethe same cross-sectional shape, but this is not necessary. Like grooves134, 144, grooves 136, 146 may have any of at least the followingcross-sections: rounded, circle segment, angular, rectangular, square,trapezoidal and triangular.

Spheres 160 may be sized to correspond with the cross-sectional shapesof grooves 136, 146. Spheres 150 and 160 may be of the same or differentsizes and materials. The size of spheres 160 may be based at least inpart on one or more of the cost of spheres 160, the shear strength ofspheres 160, the size of the gap sub and the potential presence ofdefects in spheres 160.

Spheres 160 may be made of any of the materials suitable for spheres150. In some embodiments, spheres 150 and 160 are made of the samematerial. In other embodiments spheres 150 and 160 are made of differentmaterials.

In some embodiments, spheres 150 and/or spheres 160 may be replaced bynon-spherical discrete bodies that correspond to the shape of grooves134, 144 and grooves 136, 146, respectively.

In some embodiments, mating part 138 does not have a consistent outerdiameter. Mating part 138 may comprise two or more sections wherein eachsection has a different diameter. For example, mating part 138 maycomprise a first section 138A having a first diameter and a secondsection 138B having a second diameter. First section 138A may correspondwith the section of mating part 138 into which grooves 134 are arranged.Second section 138B may comprise the section of mating part 138 intowhich grooves 136 are arranged. The diameter of first section 138A maybe greater than the diameter of second section 138B, as is illustratedin FIG. 2 . In other embodiments, the diameter of mating part 138 maytaper along its length. In other embodiments, the diameter of matingpart 138 may be the same along its length.

Similarly, mating part 148 of female part 140 may comprise correspondingsections, wherein each section has a different internal bore diameter.For example, mating part 148 may have a first section 148A having afirst internal bore diameter and a second section 148B having a secondinternal bore diameter. First section 148A may comprise the section ofmating part 148 into which grooves 144 are arranged. Second section 148Bmay comprise the section of mating part 148 into which grooves 146 arearranged. The internal bore diameter of first section 148A may begreater than the internal bore diameter of second section 148B, as isillustrated in FIG. 2 . This feature of having varying diameters may becalled the “stepped joint” feature.

The outer diameter of first section 138A may be slightly less than theinternal bore diameter of the first section 148A while the outerdiameter of the second section 138B may be slightly less than theinternal bore diameter of the second section 148B so as to create a gapbetween male part 130 and female part 140. In particular embodiments,for example, the outer diameter of first section 138A is approximately2-10 mm less than the internal bore diameter of first section 148A whilethe outer diameter of the second section 138B is approximately 2-10 mmless than the internal bore diameter of the second section 148B. The gapbetween the internal bore of first section 148A and the externaldiameter of first section 138A can be filled with a suitable highdielectric material 141 (see e.g. FIG. 3 ) as described in more detailbelow.

The stepped joint feature may simplify installation of the gap sub as itcan allow for anti-torque elements, such as spheres 160, to be installedbefore mating male part 130 to female part 140. In particularembodiments, where the internal bore diameter of section 148A is greaterthan the sum of the outer diameter of section 138B and the protrudingportions of spheres 160, it is possible to insert spheres 160 intogrooves 136 before inserting male part 130 into female part 140, therebyobviating the need for openings 154. Due to the different diameters ofsections 138A, 138B and corresponding sections 148A, 148B, spheres 160inserted into grooves 136 before mating can pass by grooves 144 as malepart 130 is inserted into female part 140.

It is advantageous to be able to install spheres 160 before insertingmale part 130 into female part 140. In this way, spheres 160 can beinstalled without the need to provide a fill port for each of grooves146 or to feed spheres 160 one-by-one into grooves 146. This reducesmanufacturing time and costs in constructing gap sub 120. Reducing oreliminating openings 154 reduces the possibility of leaking in gap sub120 and thereby increases reliability.

When inserting spheres 160 into grooves 136, it is beneficial to securethem in place around male mating part 138 before inserting male part 130into female part 140. Securing spheres 160 in place around male matingpart 138 simplifies installation because all spheres 160 can bepre-assembled and installed together as described below. Spheres 160 canbe secured in place using various apparatus and methods. For example,spheres 160 can be secured in place using an adhesive, a torsionalchannel ball ring, a torsional channel scarf or torsional channel pods,as described below.

In some embodiments, spheres 160 are secured in place in grooves 136using an adhesive. Each sphere 160 may be bonded in place in one ofgrooves 136 using an adhesive such as a suitable cement, glue or epoxy.The adhesive may be first applied to grooves 136 or to spheres 160. Inother embodiments, spheres 160 are bonded to one another using anadhesive. Alternatively, spheres 160 may be attached together using arod or wire that passes through the centers of spheres 160, like beadson a string.

In some embodiments, spheres 160 are secured in place using a torsionalchannel ball ring 164, as illustrated in FIGS. 9 and 10 . Torsionalchannel ball ring 164 may comprise a ring-shaped support structure 164Afor retaining spheres 160. Support structure 164A may comprise aninjected plastic, epoxy or another suitable material. Support structure164A comprises a plurality of channels 164B that receive spheres 160. Asillustrated in FIG. 9 , spheres 160 are retained in channels 164B andare organized in a plurality of columns. Each column corresponds to oneof the plurality of pairs of grooves 136, 146 formed into mating parts138, 148. Each of the plurality of columns of spheres 160 is spacedapart from adjacent columns by the same spacing as between adjacentgrooves 136 or adjacent grooves 146.

In some embodiments, spheres 160 are retained in channels 164B entirelyby channels 164B while in other embodiments, spheres 160 are retained inchannels 164B by a combination of grooves 136 and channels 164B orgrooves 146 and channels 164B. In particular, channels 164B may bepartially or completely open toward the interior of torsional channelball ring 164, may have an opening toward the exterior of torsionalchannel ball ring 164 or may have one or more openings along one or moreof the edges of torsional channel ball ring 164. As such, spheres 160may be inserted into channels 164B before or after torsional channelball ring 164 is installed around mating part 138. In other embodiments,spheres 160 and torsional channel ball ring 164 are together placed in amold and bonded together to create a single assembly.

Each channel 164B contains one or more spheres 160. The number ofspheres 160 in each channel 164B is less than or equal to the length ofthe groove 136, 146 divided by the diameter of a sphere 160.

Once torsional ball channel ring 164 and spheres 160 are installed ontomating part 138, mating part 138 may be inserted into mating part 148.Mating part 138 is aligned so that grooves 136 align with grooves 146 ofmating part 148. In this way, mating part 138 may be completely insertedinto mating part 148 and male part 130 is secured torsionally relativeto female part 140.

Using torsional ball channel ring 164 increases the ease and speed ofinstalling and securing male part 130 in female part 140. Sincetorsional ball channel ring 164 remains between male part 130 and femalepart 140, it may also improve electrical isolation of male part 130 fromfemale part 140 and eliminate any potential fluid paths from developingaround spheres 160.

Support structure 164A may be relatively thin. In some embodiments,support structure 164A is between 0.5 mm and 5 mm in thickness. Thethickness of support structure 164A may be constant throughout or mayvary. The diameter of spheres 160 may be reduced and/or thecross-sectional area of grooves 136, 146 may be increased to accommodatesupport structure 164A and thereby maintain appropriate clearancebetween male part 130 and female part 140. In some embodiments, supportstructure 164A is thick enough to fill the radial gap in channels 164Bwhile, in other embodiments, support structure 164A is thin enough toallow for additional injected plastic to travel the full length ofchannel 164B.

In some embodiments, spheres 160 are secured in place using a torsionalchannel ball scarf 166. Torsional channel ball scarf 166 may comprise asupport structure 166A for retaining spheres 160. Support structure 166Amay comprise a flexible, generally rectangular, strip having a pluralityof channels 166B. Torsional ball scarf 166 may be installed by wrappingit around mating part 138. Support structure 166A may comprise aninjected plastic, epoxy, woven or non-woven fabric, or another suitablematerial. Support structure 166A supports columns of spheres 160. In theillustrated embodiment, channels 166B receive spheres 160. Asillustrated in FIG. 11 , spheres 160 are arranged in channels 166B so asto provide a plurality of columns. Each column corresponds to one of theplurality of pairs of grooves 136, 146 formed into mating parts 138,148. Each of the plurality of channels 166B of spheres 160 is spacedapart from adjacent channels 166B by the same spacing as adjacentgrooves 136 or adjacent grooves 146. In other embodiments, spheres 160and torsional channel ball scarf 166 are together placed in a mold andbonded together to create a single assembly.

In some embodiments, spheres 160 are retained in channels 166B entirelyby channels 166B while in other embodiments, spheres 160 are retained inchannels 166B by a combination of grooves 136 and channels 166B. Inparticular, channels 166B may be partially or completely open toward theinterior of torsional channel ball scarf 166, may have an opening towardthe exterior or torsional channel ball scarf 166 or may have one or moreopenings along one or more of the edges of torsional channel ball scarf166. As such, spheres 160 may be inserted into channels 166B before orafter torsional channel ball scarf 166 is wrapped around mating part138.

Each channel 166B contains one or more spheres 160. The number ofspheres 160 in each channel 166B is less than or equal to the length ofthe groove 136, 146 divided by the diameter of a sphere 160.

Once torsional ball channel scarf 166 and spheres 160 are installed ontomating part 138, mating part 138 may be inserted into mating part 148 offemale part 140. Mating part 138 is aligned so that grooves 136 alignwith grooves 146 of mating part 148. In this way, mating part 138 may becompletely inserted into mating part 148 and male part 130 is securedtorsionally relative to female part 140.

Using torsional ball channel scarf 166 increases the ease and speed ofinstalling and securing male part 130 in female part 140. Sincetorsional ball channel scarf 166 remains between male part 130 andfemale part 140, it may also improve electrical isolation of male part130 from female part 140 and eliminate any potential fluid paths fromdeveloping around spheres 160.

Support structure 166A may be relatively thin and flexible so that itcan be wrapped around mating part 138. In some embodiments, supportstructure 166A is between 0.5 mm and 5 mm in thickness. The thickness ofsupport structure 166A may be constant throughout or may vary. Supportstructure 166A may include perforations or notches to improveflexibility. The diameter of spheres 160 may be reduced and/or thecross-sectional area of grooves 136, 146 may be increased to accommodatesupport structure 166A and thereby maintain appropriate clearancebetween male part 130 and female part 140. In some embodiments, supportstructure 166A is thick enough to fill the radial gap in between grooves136, 146 while in other embodiments, support structure 166A is thinenough to allow for additional injected plastic to travel the fulllength of grooves 136, 146.

In some embodiments, spheres 160 are secured in place using a pluralityof torsional channel ball pods 168, as depicted in FIGS. 13 and 14 .Each torsional ball pod 168 comprises a tubular support structure 168Afor retaining spheres 160 and corresponds to one of the plurality ofpairs of grooves 136, 146. The number of spheres 160 in each torsionalball pod is less than or equal to the length of each of grooves 136, 146divided by the diameter of spheres 160.

Support structure 168A may comprise an injected plastic, epoxy oranother suitable material. Support structure 168A has a channel 168Bthat receive spheres 160. Channel 168B may open from a lengthwise sideof support structure 168A or from an end of support structure 168A.Alternatively, support structure 168A comprises a heat-shrink materialwrapped around spheres 160 and does not include any openings. In otherembodiments, material may be molded around spheres 160. Spheres 160 areheld in place by one or more retention features. The retention featuresmay work in conjunction with one of grooves 136 or grooves 146 bysandwiching spheres 160 between the retention features and grooves 136or grooves 146.

Torsional ball channel pods 168 may be separately installed into each ofgrooves 136, as illustrated in FIG. 14 . In practice, spheres 160 may beloaded into torsional ball channel pod 168 before a torsional ballchannel pod 168 is installed on mating part 138, as depicted in FIGS. 13and 14 . Alternatively, in some embodiments, spheres 160 may be loadedinto torsional ball pods 168 after torsional ball channel pods 168 areinstalled on mating part 138.

Torsional ball channel pods 168 may be installed into grooves 136 in anysuitable manner, such as, for example, press fitting or bonding usingadhesive. Once torsional ball channel pods 168 are installed onto matingpart 138, mating part 138 may be inserted into mating part 148 of femalepart 140. Mating part 138 is aligned so that grooves 136 align withgrooves 146 of mating part 148. In this way, male part 130 is securedtorsionally relative to female part 140.

Using torsional ball channel pods 168 increases the ease and speed ofinstalling and securing male part 130 in female part 140. Sincetorsional ball channel pods 168 remain between male part 130 and femalepart 140, they may also improve electrical isolation of male part 130from female part 140 and eliminate any potential fluid paths fromdeveloping around spheres 160.

Support structure 168A may be relatively thin so that torsional ballchannel pods 168 can be installed on mating part 138. In someembodiments, support structure 168A is between 0.5 mm and 5 mm inthickness. The thickness of support structure 168A may be constantthroughout or may vary. The diameter of spheres 160 may be reducedand/or the cross-sectional area of grooves 136, 146 may be increased toaccommodate support structure 168A and thereby maintain appropriateclearance between male part 130 and female part 140. In someembodiments, support structure 168A is thick enough to fill the radialgap in between groves 136, 146 while in other embodiments, supportstructure 168A is thin enough to allow for additional injected plasticto travel the full length of grooves 136, 146.

In some embodiments, whether using torsional ball ring 164, torsionalball scarf 166 or torsional ball pods 168, spheres 160 are notcompletely encompassed within support structure 164A, 166A, 168A butprotrude past support structure 164A, 166A, 168A. In such embodiments,spheres 160 may contact one or both of grooves 136, 146 directly.

In some embodiments, channels 164B, 166B, 168B may be replaced with oneor more sleeves for containing individual spheres 160. For example, onechannel which could contain five spheres 160 could be replaced with fiveindividual sleeves that each contains a single sphere 160. The one ormore sleeves may be arranged adjacent each other so as to form a row ofsleeves that together generally correspond to the same shape as thechannel. Alternatively, the sleeves may each receive more than onesphere 160.

After male part 130 is secured torsionally within female part 140,spheres 150 may be inserted. Spheres 150 may be inserted through one ormore openings 154. As additional spheres 150 are inserted into opening154, spheres 1150 are pushed along helical space 152 until they reach anend of helical grooves 134, 144. After the desired number of spheres 150are installed, a cap or a plug may be installed in opening 154.

In some embodiments (e.g. gap sub 120), after the male part (e.g. malepart 130) is secured to the female part (e.g. female part 140), a fillermaterial may be injected in between the male and female parts to securethe spheres (e.g. spheres 150, 160) in place, fill any gaps, preventfluid invasion and improve the electrical isolation provided by the gapsub.

A high dielectric, nonconductive material, such as, but not limited to,an injectable thermoplastic or epoxy or engineered resin may be injectedinto the radial gap between the external surface of the male matingsection 138 and the internal surface of the female mating section 148.The injected dielectric material sets and electrically isolates the malemating section 138 from the female mating section 148, as well aspreventing drilling fluid from filling the radial gap. The dielectricmaterial may additionally help to attach male part 130 to female part140.

To ensure that the dielectric material electrically isolates male matingsection 138 from female mating section 148 and that any of a torsionalball ring 164, a torsional ball scarf 166 or one or more torsional ballpods 168 do not interfere with the injection of the dielectric materialone or more systems or methods can be incorporated. For example, one ormore conduits, leading to opposing sides of torsional ball ring 164,torsional ball scarf 166 or torsional ball pods 168 may be used toensure that the dielectric material is able to access the surface ofboth male mating part 138 and female mating part 148. Alternatively,torsional ball ring 164 and torsional ball scarf 166 may includechannels and apertures for allowing the injected dielectric material toflow around and through torsional ball ring 164 and torsional ball scarf166. In other embodiments, the injected dielectric material is hotenough to melt torsional ball ring 164, torsional ball scarf 166 and/ortorsional ball pods 168, and support structure 164A, 166A, 168A mixeswith the injected dielectric material to isolate male mating section 138from female mating section 148.

In some embodiments, support structure 166A or 168A comprises a cloth ofwoven or non-woven fibers such as carbon fiber or fiberglass. In suchembodiments, injection of the dielectric material may impregnate thesupport structure. The support structure may enhance the strength of theinjected dielectric material.

In some embodiments, injecting the dielectric material comprisesinjecting the dielectric material through one or more openings (such asopenings 154) and applying suction to one or more suction openings awayfrom openings 154 so as to draw the dielectric material into the spacebetween male part 130 and female part 140. Suction openings may beprovided in one or more locations at opposite ends of the gap betweenmale part 130 and female part 140. In some embodiments, dielectricmaterial is injected into gap sub 120 until superfluous dielectricmaterial spills out of the suction openings. In this way, the injecteddielectric material can fill the entire gap between male part 130 andfemale part 140.

FIGS. 15-18 depict another exemplary embodiment of a torsion resistantgap sub 220. Instead of helical grooves 134, 144, gap sub 220 has athreaded profile 234 on male part 230 and a corresponding threadedprofile 244 on the corresponding female part 240.

Threaded profile 234, 244 may be tapered, although this is notnecessary. It may be advantageous to provide a threaded profile since athreaded profile may better withstand various types of loading while inuse. Threaded profile 234, 244 requires fewer parts and may also provideeasier installation, less potential for air voids during injection(filling of gaps), simpler calculations of flow dynamics duringinjection, better sealing against fluid invasion and fewer fill portsfor potential fluid invasion and manufacturing complexity.

In some embodiments, two or more longitudinal grooves may be joinedtogether to form a single continuous groove, thereby reducing the numberof openings required to install spheres into the longitudinal grooves.For example, male part 230 includes U-shaped grooves 236 instead oflinear grooves 136. Spheres may be inserted into grooves 236 by way ofone or more openings 254 in each U-shaped groove 236. The U-shape ofeach groove 236 reduces the number of openings 254 by half, as comparedto embodiments using only straight grooves 136. In some embodiments, aplurality of U-shaped grooves 236 are provided around mating part 238,as illustrated in FIG. 16 . U-shaped grooves may alternate inorientation around the circumference of mating part 238, such as isillustrated in FIGS. 16 and 18 . In some embodiments, each U-shapedgroove is connected to adjacent U-shaped grooves to provide a singlegroove around the circumference of mating part 238 which has multiplecircumferentially spaced sections that extend generally axially. Inother embodiments, each U-shaped groove is separate and requires aseparate opening 254.

In other embodiments, U-shaped grooves 236 may be replaced by grooves ofother shapes that are interconnected and incorporate at least somelongitudinally oriented groove portions. For example, three longitudinalgrooves could be connected together to form S-shaped grooves. S-shapedgrooves would further reduce the number of openings 254 as compared toU-shaped grooves. In other embodiments, a plurality of S-shaped groovescould be interconnected to form one continuous groove with a singleopening 254 for receiving spheres.

In some embodiments, since each groove 236, 246 is separate, a separateopening 254 is provided for each groove 236, 246. After inserting thedesired number of spheres, each opening 254 is plugged using a fill plugin order to avoid invasion of fluids into gap sub 220.

In order to electrically isolate male part 230 from female part 240, ahigh dielectric, nonconductive material may be injected into a gapbetween the external surface of the male part 230 and the internalsurface of the female part 240. The injected dielectric material setsand electrically isolates male part 230 from the female part 240, aswell as preventing drilling fluid from filling the radial gap. Thedielectric material may additionally help to attach male part 230 tofemale part 240.

Another aspect of the invention provides a gap sub assemblysubstantially similar to gap sub assembly 120 except that grooves 136,146 are helical and grooves 134, 144 are longitudinal. In such anembodiment, spheres or other bodies may be installed into the helicalgrooves before the male part is inserted into the female part. Thespheres may be held in place by a ball scarf, a ball ring or a helicalball pod similar to those disclosed above except that they only have asingle helical channel for retaining the spheres. With the spheres heldin place, the male part is rotated relative to the female part as theyare mated to achieve the desired amount of pre-load compression on thecollar. After the desired amount of pre-load compression is obtained,spheres or other suitable bodies are inserted into the longitudinalchannels by way of openings to maintain the pre-load and stop relativerotation between the male and female parts.

After installing spheres (e.g. spheres 150, 160), it is typicallydesirable to close off the interior of the gap sub so as to prevent theinvasion of fluid into gap sub 120. A fill plug 180 may be inserted into an opening to secure spheres within the gap sub. Fill plug 180 may beof a corresponding size and shape to snugly fit within the opening. Insome embodiments, the opening and fill plug 180 are circular in crosssection. Fill plug 180 may comprise an internal channel for allowing adielectric material to be injected into the gap sub, as described above.After the dielectric material sets, the internal channel of fill plug180 may be blocked by the dielectric material to prevent any fluids fromentering the gap sub.

It is generally preferred that fill plug 180 comprises a soft materialthat can be deformed to provide a snug fit within a correspondingopening without damaging the opening or the gap sub generally. Fill plug180 may comprise various materials such as plastic.

Fill plug 180 may comprise one or more o-rings 182 for providing aresilient seal in an opening, as depicted in FIG. 19A. Traditionally, nolubricant is used in installing a plastic fill plug since lubricant is acontaminant that can lead to incomplete injection. However, withoutlubricant, it may be difficult to obtain a tight fit and good seal inthe opening without damaging the O-ring during installation.

Another aspect of the invention provides an intermediate plug 190 forclosing openings (e.g. openings 154, 254) and improving the seal of fillplug 180. To stop lubricant from contaminating the internal gap,intermediate plug 190 may be inserted between the last sphere and fillplug 180 to which the lubricant is applied.

Intermediate fill plug 190, like fill plug 180 may comprise an internalchannel for allowing dielectric material to be injected into the gapsub. In some embodiments, the internal channel of intermediate fill plug190 is shaped to receive and thereby extend the internal channel of fillplug 180 so that dielectric material can be injected through bothintermediate fill plug 190 and fill plug 180.

In some embodiments, intermediate plug 190 is installed before fill plug180 is installed while in other embodiments, intermediate plug 190 is anintegral part of fill plug 180. In some embodiments, intermediate fillplug 190 takes the place of the last sphere in the internal gap, whilein other embodiments, intermediate fill plug 190 envelopes a portion ofthe last sphere and secures the last sphere in place.

Intermediate plug 190 provides a barrier which blocks any lubricant fromtravelling from fill plug 180 into the internal gap, as illustrated inFIG. 19B. As such, lubricant can be applied to the o-rings 182 that formpart of fill plug 180. It is advantageous to provide lubricant wheninstalling fill plug 180 since it will ease installation and ensure thato-rings 182 are not damaged during installation, thereby providing abetter seal to stop fluid invasion into the gap sub. Intermediate plug190 may comprise plastic or another soft material so as not to damagegrooves (e.g. grooves 134, 136, 144, 146), spheres (e.g. spheres 150,160) or openings (e.g. openings 154, 254).

In some embodiments, where nonconductive material is injected in tospace 152 or space 162, intermediate fill plug 190 may have a centralaperture to allow the nonconductive material to pass through. As thenonconductive material solidifies, it may engage with locking feature192 (depicted in FIG. 19C) or locking feature 194 (depicted in FIG.19D). Locking feature 192 comprises an aperture that may be filled withnonconductive material that is injected. Once the nonconductive materialhardens, it locks intermediate fill plug 190 in place. Similarly,locking feature 194 comprises one or more notches that may be filledwith nonconductive material to lock intermediate fill plug 190 in place.

The ability of a body that spans a gap between channels or otherindentations to resist forces may be increased by configuring the bodyand/or the channels or indentations in which the body is received suchthat contact between the body and surfaces of the channels or otherindentations is distributed along a line or on a surface as opposed tobeing a point contact. In some embodiments bodies are engaged inchannels that are shaped to provide such distributed contact. In someembodiments the shaping comprises pockets or recesses formed along sidesof grooves (e.g. grooves 134, 136, 144, 146) that are shaped to conformto the surfaces of bodies received in the grooves. For example, wherethe bodies are spherical (like spheres 150, 160 for example) therecesses may have the form of a portion of a sphere of the same diameteras spheres 150, 160.

Recesses or pockets may be formed on one or both sides of a groove. Insome embodiments recesses are formed along opposing sides of grooves 134and 144 or opposing sides of grooves 136, 146.

Torque requirements on a gap sub are typically not symmetrical. Torquesthat arise as a result of drilling are generally in one direction. Therecesses in channels 136, 146 may be located such that the torques thatarise as a result of drilling tend to more fully engage spheres 160 (orother bodies in channels 136, 146) with the provided recesses.

Axial forces that a gap sub may be called upon to resist are alsotypically not symmetrical. For example, a gap sub close to a drill bitat the downhole end of a drill string may need to withstand verysignificant compressive forces (e.g. arising from the weight of thedrill string above the gap sub). The forces that tend to extend the gapsub will typically be significantly lower than the compressive forces.Consequently, the recesses in channels 134, 144 may be located such thatthe largest expected axial forces tend to more fully engage spheres 150(or other bodies in channels 134, 144) with the provided recesses.

Recesses or pockets may be provided along one or both edges of one ormore grooves in any of the embodiments described herein. In someembodiments a process for making a gap sub comprises applying forces tomale and female parts 130, 140 which tend to seat bodies (e.g. spheres150 and/or 160) in corresponding recesses or pockets while injecting asettable dielectric material into the gap. In some embodiments theforces comprise a torque. In some embodiments the forces comprise both atorque and an axial force. In some embodiments the forces comprise anaxial force.

Recesses or pockets may be formed to receive bodies having shapes otherthan spherical shapes. For example, pockets may be shaped to receivecylindrical or barrel-shaped bodies.

It is not necessary that pockets or recesses be formed in all channels.In some embodiments some channels comprise pockets or recesses asdescribed above and other channels do not. For example, 1, 2, or morechannels comprising pockets or recesses may be provided between eachpair of adjacent channels that do not comprise pockets or recesses. Inembodiments where not all channels comprise pockets or recessescorresponding channels on male and female parts 130, 140 that do notcomprise pockets or recesses may be positioned such that they areproperly aligned to receive bodies (e.g. spherical balls 160) whenbodies in the other channels that do comprise recesses or pockets arefully engaged in the recesses or pockets. Inserting bodies in thechannels that do not comprise recesses or pockets retains the bodies inthe other channels that do comprise recesses or pockets such that thosebodies are kept fully engaged in the recesses or pockets.

A method for making a gap sub in an embodiment where some of channels136, 146 include pockets or channels and some do not may comprise:inserting bodies into those channels that do include pockets orrecesses; twisting male part 130 relative to female part 140 such thatthe bodies are each received in corresponding pockets or recesses in thesides of corresponding channels 136, 146 and such that correspondingchannels 136, 146 that do not include pockets or recesses are alignedwith one another; and inserting bodies into the channels 136, 146 thatdo not include pockets or recesses.

A method for making a gap sub in an embodiment where some of channels134, 144 include pockets or channels and some do not may comprise:inserting bodies into those channels 134, 144 that do include pockets orrecesses; moving male part 130 axially relative to female part 140 suchthat the bodies are each received in a corresponding pocket or recess inthe sides of corresponding channels 134, 144 and such that correspondingchannels 134, 144 that do not include pockets or recesses are alignedwith one another; and inserting bodies into the channels 134, 144 thatdo not include pockets or recesses.

FIG. 20A is a schematic view showing a portion of an exemplary male part130 comprising channels 136A that do include pockets 137 that arecup-shaped to receive portions of balls 160 alternating with channels136B that do not include pockets 137. FIG. 20B is a schematic viewshowing a portion of an exemplary female part 140 comprising channels146A that do include pockets 137 that are cup-shaped to receive portionsof balls 160 alternating with channels 146B that do not include pockets137.

FIG. 20C is a cross-section showing a portion of an exemplary gap sub200 comprising male and female parts 130, 140 with channels 136A, 146Ahaving pockets 137 and channels 136B, 146B without pockets 137. Malepart 130 is positioned relative to female part 140 such that balls 160can be inserted into channels 136A, 146A. Channels 136B, 146B aremisaligned. Rotating male part 130 relative to female part 140 indirection D1 or rotating female part 140 relative to male part 130 indirection D2 fully engages balls 160 with pockets 137 and alignschannels 136B, 146B.

FIG. 20D illustrates a cross-section showing a portion of exemplary gapsub 200 comprising channels 136A, 136B, 146A, 146B engaged with balls160.

Providing spherical balls as the bodies that retain and maintain spacebetween male and female parts 130, 140 is convenient because sphericalballs do not have a required orientation and can be pushed alongchannels that have variable curvature. However, bodies having othershapes are also advantageous.

For straight sections of channels (e.g. channels 136, 146) the bodiesmay be cylindrical or have other shapes in which the body has a constantcross-sectional shape (e.g. such bodies may have the form of rods thathave cross sections that are regular hexagons, other hexagonal shapes,square, rectangular, circular, oval, etc.). The channels which receivesuch bodies may have sides that conform to the side faces of the bodies.This can provide at least a line of contact between the body and each ofthe grooves with which it is engaged. Such bodies may providesignificantly greater bearing face surface area than a group ofspherical balls occupying the same section of a groove.

For sections of channels that have a constant curvature (e.g. channels134, 144) bodies may be provided in the form of cylindrical segmentsbent to match the curvature of the channels or in the form ofbarrel-shaped segments, for example. Such bodies may be pushed along thechannels in much the same way as spherical balls.

While such bodies may have the form of elongated rods it is advantageousfor the bodies to be provided in relatively short sections. In someembodiments the bodies comprise a plurality of short segments that fittogether end-to-end to form elongated rods. In some embodiments thelength of each section is no more than 2 to 6 times greater than thesize of the gap between the surfaces in which channels 136, 146 areformed. Providing bodies on the form of such short segments can tend tolimit the propagation of cracks and can tend to reduce the likelihoodthat the bodies will be cracked or otherwise damaged by theloads/bending experienced when the gap sub is in use.

FIG. 21A is a perspective view of a male part 130 in an exampleembodiment wherein bodies 250 have the form of segmented rods having asquare cross-section. In the illustrated embodiment individual segments270A of bodies 250 are in the form of cubes. FIG. 21B is a perspectiveview of a male part 130 in an example embodiment wherein bodies 250 havethe form of segmented rods having a round cross-section. In theillustrated embodiment individual segments 271A of bodies 250 are in theform of small cylinders.

In some embodiments the bodies are in the form of rods that arepre-formed to have spaced-apart weak sections such that the rods can beinserted as single components but will snap at the weak sections intoseparate segments if exposed to sufficient forces in use.

FIGS. 22A through 22F are schematic cross sections through a pluralityof example bodies and grooves, FIG. 22A shows a cylindrical body 250Aengaged between arcuate grooves. FIG. 22B shows a cylindrical body 250Aengaged between V-shaped grooves. FIG. 22C shows a square body 250Creceived between rectangular grooves. FIG. 22D shows a hexagonal body250D received between trapezoidal grooves. FIG. 22E shows an ellipticalbody 250E engaged between partial elliptical grooves. It is notmandatory that the cross-sectional shapes of the bodies are convex. Forexample, FIG. 22F shows a bowtie-shaped body 250F engaged betweendovetail grooves. In the embodiment of FIG. 22F body 250F is slidaxially into the grooves (either one at a time or simultaneously).

Where the bodies used have non-circular cross-sections, the bodies maybe oriented to improve the abilities of the bodies to resist forcesapplied between the male and female parts. In example embodiments alonger dimension of the cross section is aligned at an angle to theradial line passing from a longitudinal centerline of the male partthrough a center of the body. An example embodiment having thisconstruction is illustrated in FIG. 23 which is a cross section througha gap sub. Bodies 250 have elliptical cross-sections. Each of bodies 250spans between a groove 136 on male part 130 and a corresponding groove146 on female part 140. Grooves 136 and 146 have cross-sectional shapesthat substantially conform to the received portion of body 250. Grooves136, 146 are formed in such a way that at least some of bodies 250 areheld such that the major transverse axis of the body does not coincidewith a radius of the gap sub.

In the illustrated embodiment the elliptical cross section of at leastsome of bodies 250 (identified as 250-1) is aligned such that the majortransverse axis T1 of the body is aligned at an angle α to a radial lineR1 passing through the center of the body 250. The elliptical crosssection of some other ones of bodies 250 (identified as 250-2) isaligned such that the major transverse axis T2 of the body is aligned atan angle β to a radial line R2 passing through the center of the body250. In some embodiments bodies 250-1 alternate with bodies 250-2.

Angles α and β may be the same or different in magnitude. Advantageouslyangles α and β are opposite in sign such that bodies 250-1 are bestpositioned to resist torque that tends to turn female part 140 in afirst direction relative to male part 130 and bodies 250-2 are bestpositioned to resist torque that tends to turn female part 140 in asecond direction opposite to the first direction relative to male part130.

Bodies 250 may comprise rods or rows of segments having cross-sectionsas shown in FIG. 22 . Bodies 250 may have other cross-sectional shapes(e.g. rectangular, obround, oval, etc.) which permit a longer transversedimension of the bodies to be oriented to best resist forces appliedbetween male and female parts 130, 140.

Bodies in the form of rods or non-spherical segments may be inserteddirectly into a gap sub or may be provided in the form of pods, scarvesor rings in substantially the same manner as described above (seedescription of ring 164, scarf 166 and pods 168 above). This can improveefficiency for making a gap sub by reducing the number of parts thatmust be handled individually.

In some embodiments the surfaces on either side of one or more groovesor other recesses in which bodies are received are formed such thatopposing edges of the grooves have different elevations. Thisconstruction can be advantageous in cases where the bodies are expectedto resist forces that are larger in one direction than another as it canprovide greater area of contact between a body and the male and femaleparts 130, 140 while maintaining a desired gap between male and femaleparts 130, 140. Any of the embodiments described herein may be modifiedto provide this feature. Elevation of surfaces on the male part may bemeasured radially outwardly from a longitudinal centerline of the malepart. Elevation of surfaces on the female part may be measured radiallyinwardly from a longitudinal centerline of the female part.

FIG. 24 is a cross section through a body 250 in an example embodimentwherein the body (which in the illustrated embodiment may be in the formof a sphere or rod) spans a gap between a groove 136-1 in male part 130and a groove 146-1 in female part 140. In this embodiment groove 146-1extends between surfaces 240L and 240R on female part 140 and groove136-1 extends between surfaces 230L and 230R on male part 130. Surface240L projects more from female part 140 than surface 240R. Surface 230Rprojects more from male part 130 than surface 230L. This constructionallows body 250 to better resist forces in the direction of arrow 231.

In some embodiments a centroid of the cross-section of at least one ofthe bodies that spans between the grooves of a pair of correspondinggrooves (e.g. 136-1 and 146-1) is radially between the higher side ofthe groove on the male mating section and the higher side of the grooveon the female mating section.

There are several ways in which the elevations of surfaces adjacent todifferent grooves may be arranged. In different embodiments one mayencounter a different pattern of lower and higher edges as one travelsin a direction transverse to a set of grooves (e.g. grooves 134, 144,136 or 146). The path travelled along may, for example, extend on acircumferential line following the surface of the male or female part ora longitudinal line extending along the surface of the male or femalepart.

For example in one embodiment one may encounter a first side of a firstgroove that is higher in elevation, cross the first groove, encounter asecond side of the first groove that is lower in elevation, travel untila first side of a second groove that is higher in elevation, cross thesecond groove to a second side of the second groove that is lower inelevation, and so-on. In other words, the pattern of higher and lowersides of the grooves in this example may be represented as H-L, H-L, H-L. . . and so on where the hyphens represent grooves and the commasrepresent surfaces between grooves. The same pattern could berepresented as L-H, L-H, L-H . . . when travelling along the path in theopposite direction. Patterns of higher and lower sides of correspondinggrooves on the male and female parts may be complementary to one another(e.g. the higher side of a groove on the male part is generally alignedwith the lower side of the corresponding groove on the female part andvice versa. Such embodiments may be beneficial where forces exertedparallel to the path are expected to be much larger in one directionthan in the opposing direction.

In another example embodiment the pattern may be represented as H-L,L-H, H-L, L-H . . . and so on. FIG. 25 is a cross section through anexample gap sub having this construction. In the illustrated embodiment,one may encounter a first side 336-1H of groove 336-1 that is higher inelevation than a second side 336-1L, followed by second side 336-1L,followed by a first side 336-2L of groove 336-2 that is lower inelevation than a second side 336-2H, followed by second side 336-2H,followed by a first side 336-3H of groove 336-3 that is higher inelevation than a second side 336-3L, followed by second side 336-3L,followed by a first side 336-4L of groove 336-4 that is lower inelevation than a second side 336-4H, followed by second side 336-4H . .. and so on. Such embodiments may beneficially increase capacity forresisting forces between the male and female parts compared to similarembodiments where the sides of grooves are equal in elevation. Otherembodiments are also possible. In any embodiments where the elevationson opposing sides of a groove differ, surfaces between adjacent groovesmay ramp, curve or step to accommodate any difference in elevationbetween the side of one groove and the nearest side of an adjacentgroove.

Some embodiments in which the elevations on opposing sides of a groovediffer also provide recesses or pockets to receive bodies as describedabove. In such embodiments the recesses or pockets may advantageously beprovided on the wall of the groove that is on the higher side of thegroove.

In some embodiments the high side of a groove on the female partradially overlaps with the high side of the corresponding groove on themale part. In such embodiments, forces that attempt to move the malepart relative to the female part in a direction that would bring thehigh sides of these grooves closer together can be resisted by a bodyspanning between these grooves in a way that results on compression ofthe body as opposed to shear of the body.

It is not mandatory for corresponding grooves parts (e.g. grooves 134,144 or grooves 136, 146) on the male and female parts 130, 140 toreceive a body to the same depth. In some embodiments a groove in malepart 130 is deeper than the corresponding groove in female part 140 suchthat a body (e.g. a spherical ball) projects into the groove of malepart 130 more deeply below the surrounding surface of male part 130 thanthe body projects into the groove of female part 140. FIG. 26 shows anexample embodiment wherein grooves 136 on male part 130 are deeper thancorresponding grooves 146 on female part 140. This construction mayprovide enhanced torque resistance.

In other embodiments the bodies and corresponding grooves or otherrecesses are shaped such that a surface area of contact between the bodyand the male part is greater than the surface area of contact betweenthe body and the female part.

Interpretation of Terms

Unless the context clearly requires otherwise, throughout thedescription and the claims:

-   -   “comprise,” “comprising,” and the like are to be construed in an        inclusive sense, as opposed to an exclusive or exhaustive sense;        that is to say, in the sense of “including, but not limited to”.    -   “connected,” “coupled,” or any variant thereof, means any        connection or coupling, either direct or indirect, between two        or more elements; the coupling or connection between the        elements can be physical, logical, or a combination thereof.    -   “herein,” “above,” “below,” and words of similar import, when        used to describe this specification shall refer to this        specification as a whole and not to any particular portions of        this specification.    -   “or,” in reference to a list of two or more items, covers all of        the following interpretations of the word: any of the items in        the list, all of the items in the list, and any combination of        the items in the list.    -   the singular forms “a,” “an,” and “the” also include the meaning        of any appropriate plural forms.

Words that indicate directions such as “vertical,” “transverse,”“horizontal,” “upward,” “downward,” “forward,” “backward,” “inward,”“left,” “right,” “front,” “back,” “top,” “bottom,” “below,” “above,”“under,” and the like, used in this description and any accompanyingclaims (where present) depend on the specific orientation of theapparatus described and illustrated. The subject matter described hereinmay assume various alternative orientations. Accordingly, thesedirectional terms are not strictly defined and should not be interpretednarrowly.

Where a component (e.g. a body, assembly, device, drill stringcomponent, drill rig system, etc.) is referred to above, unlessotherwise indicated, reference to that component (including a referenceto a “means”) should be interpreted as including as equivalents of thatcomponent any component which performs the function of the describedcomponent (i.e., that is functionally equivalent), including componentswhich are not structurally equivalent to the disclosed structure whichperforms the function in the illustrated exemplary embodiments of theinvention.

Specific examples of systems, methods and apparatus have been describedherein for purposes of illustration. These are only examples. Thetechnology provided herein can be applied to systems other than theexample systems described above. Many alterations, modifications,additions, omissions and permutations are possible within the practiceof this invention. This invention includes variations on describedembodiments that would be apparent to the skilled addressee, includingvariations obtained by: replacing features, elements and/or acts withequivalent features, elements and/or acts; mixing and matching offeatures, elements and/or acts from different embodiments; combiningfeatures, elements and/or acts from embodiments as described herein withfeatures, elements and/or acts of other technology; and/or omittingcombining features, elements and/or acts from described embodiments.

It is therefore intended that the following appended claims and claimshereafter introduced are interpreted to include all such modifications,permutations, additions, omissions and sub-combinations as mayreasonably be inferred. The scope of the claims should not be limited bythe preferred embodiments set forth in the examples, but should be giventhe broadest interpretation consistent with the description as a whole.

What is claimed is:
 1. A gap sub comprising: a female part having afemale mating section; a male part electrically isolated from the femalepart, the male part having a male mating section and a gap section, themale mating section being inserted into a bore of the female matingsection whereby the male and female mating sections overlap in an axialdirection and are spaced radially apart from one another to provide aradial gap between the male and female mating sections wherein surfacesof the male and female mating sections facing one another across theradial gap comprise grooves, grooves on the surface of the male matingsection corresponding to grooves on the surface of the female matingsection, forming pairs of corresponding grooves; an electricallyinsulating collar positioned on the gap section; and a plurality ofelectrically-insulating bodies located between the overlapping male andfemale mating sections and spanning between each pair of correspondinggrooves in the surfaces of the male and female mating sections such thatthe male and female parts are mechanically coupled together; wherein thesurfaces of the male and female mating sections are formed such thatopposing edges of grooves of at least one pair of corresponding grooveshave different elevations; and wherein, for the at least one pair ofcorresponding grooves, a higher edge of the groove on the male matingsection is radially outward relative to a higher edge of thecorresponding groove on the female mating section.
 2. A gap subaccording to claim 1, wherein a centroid of at least one of the bodiesthat spans between the at least one pair of corresponding grooves isradially between the higher edge of the groove on the male matingsection and the higher edge of the groove on the female mating section.3. A gap sub according to claim 1, wherein, for at least one pair ofcorresponding grooves the higher edge and a lower edge of the groove onthe male mating section are respectively radially aligned with a loweredge and the higher edge of the corresponding groove on the femalemating section.
 4. A gap sub according to claim 1, wherein the bodiescomprise spherical balls.
 5. A gap sub according to claim 1, wherein thebodies are cylindrical.
 6. A gap sub according to claim 1, wherein thebodies comprise elongated rods.
 7. A gap sub according to claim 6,wherein the rods are segmented.
 8. A gap sub according to claim 6,wherein the rods are non-circular in cross-section.
 9. A gap subaccording to claim 8, wherein the rods are polygonal in cross section.10. A gap sub according to claim 1, wherein at least one pair ofcorresponding grooves extends substantially longitudinally along the gapsub.
 11. A gap sub according to claim 10, wherein higher and lower edgesof the longitudinal grooves on the male mating section are arranged suchthat, for two adjacent grooves, the higher edges are adjacent or thelower edges are adjacent.
 12. A gap sub according to claim 10, whereinhigher and lower edges of the longitudinal grooves on the male matingsection are arranged such that, for two adjacent grooves, the higheredge of one of the adjacent grooves is adjacent to the lower edges ofthe other groove.
 13. A gap sub according to claim 1, wherein at leastone of the pairs of corresponding grooves comprise pockets formed in onewall of one of the grooves, the pockets shaped to conform with a shapeof the bodies.
 14. A gap sub according to claim 1, wherein a bottomsurface of at least one of the corresponding grooves is arcuate.
 15. Agap sub according to claim 1, wherein at least one of the correspondinggrooves is V-shaped in cross section.
 16. A gap sub according to claim1, comprising a longitudinal bore extending through the gap sub andthreaded couplings on opposing ends of the gap sub.
 17. A gap subaccording to claim 16, wherein one of the threaded couplings is a pincoupling comprising a first thread and one of the threaded couplings isa box coupling comprising a second thread.
 18. A gap sub comprising: afemale part having a female mating section; a male part electricallyisolated from the female part, the male part having a male matingsection and a gap section, the male mating section being inserted into abore of the female mating section whereby the male and female matingsections overlap in an axial direction and are spaced radially apartfrom one another to provide a radial gap between the male and femalemating sections wherein surfaces of the male and female mating sectionsfacing one another across the radial gap comprise grooves, the grooveson the surface of the male mating section corresponding to the grooveson the surface of the female mating section, forming pairs ofcorresponding grooves; an electrically insulating collar positioned onthe gap section; and a plurality of electrically-insulating bodieslocated between the overlapping male and female mating sections andspanning between each pair of corresponding grooves in the surfaces ofthe male and female mating sections such that the male and female partsare mechanically coupled together; wherein at least one of the surfacesof the male and female mating sections is formed such that opposingedges of at least one of the corresponding grooves have differentelevations; and wherein at least one groove of at least one of the pairsof corresponding grooves comprises spherical cups formed in one wall ofthe at least one groove, the shape of the bodies shaped to conform withthe spherical cups.
 19. A gap sub comprising: a female part having afemale mating section; a male part electrically isolated from the femalepart, the male part having a male mating section and a gap section, themale mating section being inserted into a bore of the female matingsection whereby the male and female mating sections overlap in an axialdirection and are spaced radially apart from one another to provide aradial gap between the male and female mating sections wherein surfacesof the male and female mating sections facing one another across theradial gap comprise grooves, the grooves on the surface of the malemating section corresponding to the grooves on the surface of the femalemating section, forming pairs of corresponding grooves; an electricallyinsulating collar positioned on the gap section; and a plurality ofelectrically-insulating bodies located between the overlapping male andfemale mating sections and spanning between each pair of correspondinggrooves in the surfaces of the male and female mating sections such thatthe male and female parts are mechanically coupled together; wherein atleast one of the surfaces of the male and female mating sections isformed such that opposing edges of at least one of the correspondinggrooves have different elevations; and wherein, at least one body of theplurality of bodies projects more deeply into the respective groove thaninto the respective corresponding groove of one of the pairs ofcorresponding grooves.
 20. A gap sub according to claim 19, wherein thebody projects more deeply into the respective groove on the male matingsection than into the respective corresponding groove on the femalemating section.